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1805 lines
62 KiB
Plaintext
1805 lines
62 KiB
Plaintext
======================
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Database API reference
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======================
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Once you've created your `data models`_, Django automatically gives you a
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database-abstraction API that lets you create, retrieve, update and delete
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objects. This document explains that API.
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.. _`data models`: ../model_api/
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Throughout this reference, we'll refer to the following models, which comprise
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a weblog application::
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class Blog(models.Model):
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name = models.CharField(maxlength=100)
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tagline = models.TextField()
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def __str__(self):
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return self.name
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class Author(models.Model):
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name = models.CharField(maxlength=50)
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email = models.URLField()
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def __str__(self):
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return self.name
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class Entry(models.Model):
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blog = models.ForeignKey(Blog)
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headline = models.CharField(maxlength=255)
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body_text = models.TextField()
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pub_date = models.DateTimeField()
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authors = models.ManyToManyField(Author)
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def __str__(self):
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return self.headline
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Creating objects
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================
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To represent database-table data in Python objects, Django uses an intuitive
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system: A model class represents a database table, and an instance of that
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class represents a particular record in the database table.
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To create an object, instantiate it using keyword arguments to the model class,
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then call ``save()`` to save it to the database.
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You import the model class from wherever it lives on the Python path, as you
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may expect. (We point this out here because previous Django versions required
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funky model importing.)
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Assuming models live in a file ``mysite/blog/models.py``, here's an example::
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from mysite.blog.models import Blog
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b = Blog(name='Beatles Blog', tagline='All the latest Beatles news.')
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b.save()
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This performs an ``INSERT`` SQL statement behind the scenes. Django doesn't hit
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the database until you explicitly call ``save()``.
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The ``save()`` method has no return value.
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To create an object and save it all in one step see the `create`__ method.
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__ `create(**kwargs)`_
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Auto-incrementing primary keys
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------------------------------
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If a model has an ``AutoField`` -- an auto-incrementing primary key -- then
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that auto-incremented value will be calculated and saved as an attribute on
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your object the first time you call ``save()``.
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Example::
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b2 = Blog(name='Cheddar Talk', tagline='Thoughts on cheese.')
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b2.id # Returns None, because b doesn't have an ID yet.
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b2.save()
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b2.id # Returns the ID of your new object.
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There's no way to tell what the value of an ID will be before you call
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``save()``, because that value is calculated by your database, not by Django.
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(For convenience, each model has an ``AutoField`` named ``id`` by default
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unless you explicitly specify ``primary_key=True`` on a field. See the
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`AutoField documentation`_.)
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.. _AutoField documentation: ../model_api/#autofield
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Explicitly specifying auto-primary-key values
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If a model has an ``AutoField`` but you want to define a new object's ID
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explicitly when saving, just define it explicitly before saving, rather than
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relying on the auto-assignment of the ID.
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Example::
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b3 = Blog(id=3, name='Cheddar Talk', tagline='Thoughts on cheese.')
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b3.id # Returns 3.
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b3.save()
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b3.id # Returns 3.
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If you assign auto-primary-key values manually, make sure not to use an
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already-existing primary-key value! If you create a new object with an explicit
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primary-key value that already exists in the database, Django will assume
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you're changing the existing record rather than creating a new one.
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Given the above ``'Cheddar Talk'`` blog example, this example would override
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the previous record in the database::
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b4 = Blog(id=3, name='Not Cheddar', tagline='Anything but cheese.')
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b4.save() # Overrides the previous blog with ID=3!
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See _`How Django knows to UPDATE vs. INSERT`, below, for the reason this
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happens.
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Explicitly specifying auto-primary-key values is mostly useful for bulk-saving
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objects, when you're confident you won't have primary-key collision.
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Saving changes to objects
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=========================
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To save changes to an object that's already in the database, use ``save()``.
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Given a ``Blog`` instance ``b5`` that has already been saved to the database,
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this example changes its name and updates its record in the database::
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b5.name = 'New name'
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b5.save()
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This performs an ``UPDATE`` SQL statement behind the scenes. Django doesn't hit
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the database until you explicitly call ``save()``.
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The ``save()`` method has no return value.
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How Django knows to UPDATE vs. INSERT
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-------------------------------------
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You may have noticed Django database objects use the same ``save()`` method
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for creating and changing objects. Django abstracts the need to use ``INSERT``
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or ``UPDATE`` SQL statements. Specifically, when you call ``save()``, Django
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follows this algorithm:
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* If the object's primary key attribute is set to a value that evaluates to
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``True`` (i.e., a value other than ``None`` or the empty string), Django
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executes a ``SELECT`` query to determine whether a record with the given
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primary key already exists.
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* If the record with the given primary key does already exist, Django
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executes an ``UPDATE`` query.
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* If the object's primary key attribute is *not* set, or if it's set but a
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record doesn't exist, Django executes an ``INSERT``.
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The one gotcha here is that you should be careful not to specify a primary-key
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value explicitly when saving new objects, if you cannot guarantee the
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primary-key value is unused. For more on this nuance, see
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"Explicitly specifying auto-primary-key values" above.
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Retrieving objects
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==================
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To retrieve objects from your database, you construct a ``QuerySet`` via a
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``Manager`` on your model class.
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A ``QuerySet`` represents a collection of objects from your database. It can
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have zero, one or many *filters* -- criteria that narrow down the collection
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based on given parameters. In SQL terms, a ``QuerySet`` equates to a ``SELECT``
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statement, and a filter is a limiting clause such as ``WHERE`` or ``LIMIT``.
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You get a ``QuerySet`` by using your model's ``Manager``. Each model has at
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least one ``Manager``, and it's called ``objects`` by default. Access it
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directly via the model class, like so::
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Blog.objects # <django.db.models.manager.Manager object at ...>
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b = Blog(name='Foo', tagline='Bar')
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b.objects # AttributeError: "Manager isn't accessible via Blog instances."
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(``Managers`` are accessible only via model classes, rather than from model
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instances, to enforce a separation between "table-level" operations and
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"record-level" operations.)
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The ``Manager`` is the main source of ``QuerySets`` for a model. It acts as a
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"root" ``QuerySet`` that describes all objects in the model's database table.
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For example, ``Blog.objects`` is the initial ``QuerySet`` that contains all
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``Blog`` objects in the database.
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Retrieving all objects
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----------------------
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The simplest way to retrieve objects from a table is to get all of them.
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To do this, use the ``all()`` method on a ``Manager``.
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Example::
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all_entries = Entry.objects.all()
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The ``all()`` method returns a ``QuerySet`` of all the objects in the database.
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(If ``Entry.objects`` is a ``QuerySet``, why can't we just do ``Entry.objects``?
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That's because ``Entry.objects``, the root ``QuerySet``, is a special case
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that cannot be evaluated. The ``all()`` method returns a ``QuerySet`` that
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*can* be evaluated.)
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Filtering objects
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-----------------
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The root ``QuerySet`` provided by the ``Manager`` describes all objects in the
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database table. Usually, though, you'll need to select only a subset of the
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complete set of objects.
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To create such a subset, you refine the initial ``QuerySet``, adding filter
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conditions. The two most common ways to refine a ``QuerySet`` are:
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``filter(**kwargs)``
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Returns a new ``QuerySet`` containing objects that match the given lookup
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parameters.
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``exclude(**kwargs)``
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Returns a new ``QuerySet`` containing objects that do *not* match the given
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lookup parameters.
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The lookup parameters (``**kwargs`` in the above function definitions) should
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be in the format described in `Field lookups`_ below.
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For example, to get a ``QuerySet`` of blog entries from the year 2006, use
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``filter()`` like so::
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Entry.objects.filter(pub_date__year=2006)
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(Note we don't have to add an ``all()`` -- ``Entry.objects.all().filter(...)``.
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That would still work, but you only need ``all()`` when you want all objects
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from the root ``QuerySet``.)
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Chaining filters
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~~~~~~~~~~~~~~~~
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The result of refining a ``QuerySet`` is itself a ``QuerySet``, so it's
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possible to chain refinements together. For example::
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Entry.objects.filter(
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headline__startswith='What').exclude(
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pub_date__gte=datetime.now()).filter(
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pub_date__gte=datetime(2005, 1, 1))
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...takes the initial ``QuerySet`` of all entries in the database, adds a
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filter, then an exclusion, then another filter. The final result is a
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``QuerySet`` containing all entries with a headline that starts with "What",
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that were published between January 1, 2005, and the current day.
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Filtered QuerySets are unique
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-----------------------------
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Each time you refine a ``QuerySet``, you get a brand-new ``QuerySet`` that is
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in no way bound to the previous ``QuerySet``. Each refinement creates a
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separate and distinct ``QuerySet`` that can be stored, used and reused.
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Example::
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q1 = Entry.objects.filter(headline__startswith="What")
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q2 = q1.exclude(pub_date__gte=datetime.now())
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q3 = q1.filter(pub_date__gte=datetime.now())
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These three ``QuerySets`` are separate. The first is a base ``QuerySet``
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containing all entries that contain a headline starting with "What". The second
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is a subset of the first, with an additional criteria that excludes records
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whose ``pub_date`` is greater than now. The third is a subset of the first,
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with an additional criteria that selects only the records whose ``pub_date`` is
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greater than now. The initial ``QuerySet`` (``q1``) is unaffected by the
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refinement process.
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QuerySets are lazy
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------------------
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``QuerySets`` are lazy -- the act of creating a ``QuerySet`` doesn't involve
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any database activity. You can stack filters together all day long, and Django
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won't actually run the query until the ``QuerySet`` is *evaluated*.
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When QuerySets are evaluated
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You can evaluate a ``QuerySet`` in the following ways:
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* **Iteration.** A ``QuerySet`` is iterable, and it executes its database
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query the first time you iterate over it. For example, this will print
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the headline of all entries in the database::
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for e in Entry.objects.all():
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print e.headline
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* **Slicing.** As explained in `Limiting QuerySets`_ below, a ``QuerySet``
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can be sliced, using Python's array-slicing syntax. Usually slicing a
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``QuerySet`` returns another (unevaluated )``QuerySet``, but Django will
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execute the database query if you use the "step" parameter of slice
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syntax.
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* **repr().** A ``QuerySet`` is evaluated when you call ``repr()`` on it.
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This is for convenience in the Python interactive interpreter, so you can
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immediately see your results when using the API interactively.
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* **len().** A ``QuerySet`` is evaluated when you call ``len()`` on it.
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This, as you might expect, returns the length of the result list.
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Note: *Don't* use ``len()`` on ``QuerySet``\s if all you want to do is
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determine the number of records in the set. It's much more efficient to
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handle a count at the database level, using SQL's ``SELECT COUNT(*)``,
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and Django provides a ``count()`` method for precisely this reason. See
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``count()`` below.
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* **list().** Force evaluation of a ``QuerySet`` by calling ``list()`` on
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it. For example::
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entry_list = list(Entry.objects.all())
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Be warned, though, that this could have a large memory overhead, because
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Django will load each element of the list into memory. In contrast,
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iterating over a ``QuerySet`` will take advantage of your database to
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load data and instantiate objects only as you need them.
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Limiting QuerySets
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------------------
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Use Python's array-slicing syntax to limit your ``QuerySet`` to a certain
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number of results. This is the equivalent of SQL's ``LIMIT`` and ``OFFSET``
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clauses.
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For example, this returns the first 5 objects (``LIMIT 5``)::
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Entry.objects.all()[:5]
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This returns the fifth through tenth objects (``OFFSET 5 LIMIT 5``)::
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Entry.objects.all()[5:10]
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Generally, slicing a ``QuerySet`` returns a new ``QuerySet`` -- it doesn't
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evaluate the query. An exception is if you use the "step" parameter of Python
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slice syntax. For example, this would actually execute the query in order to
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return a list of every *second* object of the first 10::
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Entry.objects.all()[:10:2]
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To retrieve a *single* object rather than a list
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(e.g. ``SELECT foo FROM bar LIMIT 1``), use a simple index instead of a
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slice. For example, this returns the first ``Entry`` in the database, after
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ordering entries alphabetically by headline::
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Entry.objects.order_by('headline')[0]
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This is roughly equivalent to::
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Entry.objects.order_by('headline')[0:1].get()
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Note, however, that the first of these will raise ``IndexError`` while the
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second will raise ``DoesNotExist`` if no objects match the given criteria.
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QuerySet methods that return new QuerySets
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------------------------------------------
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Django provides a range of ``QuerySet`` refinement methods that modify either
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the types of results returned by the ``QuerySet`` or the way its SQL query is
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executed.
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``filter(**kwargs)``
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~~~~~~~~~~~~~~~~~~~~
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Returns a new ``QuerySet`` containing objects that match the given lookup
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parameters.
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The lookup parameters (``**kwargs``) should be in the format described in
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`Field lookups`_ below. Multiple parameters are joined via ``AND`` in the
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underlying SQL statement.
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``exclude(**kwargs)``
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~~~~~~~~~~~~~~~~~~~~~
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Returns a new ``QuerySet`` containing objects that do *not* match the given
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lookup parameters.
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The lookup parameters (``**kwargs``) should be in the format described in
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`Field lookups`_ below. Multiple parameters are joined via ``AND`` in the
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underlying SQL statement, and the whole thing is enclosed in a ``NOT()``.
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This example excludes all entries whose ``pub_date`` is the current date/time
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AND whose ``headline`` is "Hello"::
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Entry.objects.exclude(pub_date__gt=datetime.date(2005, 1, 3), headline='Hello')
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In SQL terms, that evaluates to::
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SELECT ...
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WHERE NOT (pub_date > '2005-1-3' AND headline = 'Hello')
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This example excludes all entries whose ``pub_date`` is the current date/time
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OR whose ``headline`` is "Hello"::
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Entry.objects.exclude(pub_date__gt=datetime.date(2005, 1, 3)).exclude(headline='Hello')
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In SQL terms, that evaluates to::
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SELECT ...
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WHERE NOT pub_date > '2005-1-3'
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AND NOT headline = 'Hello'
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Note the second example is more restrictive.
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``order_by(*fields)``
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~~~~~~~~~~~~~~~~~~~~~
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By default, results returned by a ``QuerySet`` are ordered by the ordering
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tuple given by the ``ordering`` option in the model's ``Meta``. You can
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override this on a per-``QuerySet`` basis by using the ``order_by`` method.
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Example::
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Entry.objects.filter(pub_date__year=2005).order_by('-pub_date', 'headline')
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The result above will be ordered by ``pub_date`` descending, then by
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``headline`` ascending. The negative sign in front of ``"-pub_date"`` indicates
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*descending* order. Ascending order is implied. To order randomly, use ``"?"``,
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like so::
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Entry.objects.order_by('?')
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To order by a field in a different table, add the other table's name and a dot,
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like so::
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Entry.objects.order_by('blogs_blog.name', 'headline')
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There's no way to specify whether ordering should be case sensitive. With
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respect to case-sensitivity, Django will order results however your database
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backend normally orders them.
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``distinct()``
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~~~~~~~~~~~~~~
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Returns a new ``QuerySet`` that uses ``SELECT DISTINCT`` in its SQL query. This
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eliminates duplicate rows from the query results.
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By default, a ``QuerySet`` will not eliminate duplicate rows. In practice, this
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is rarely a problem, because simple queries such as ``Blog.objects.all()``
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don't introduce the possibility of duplicate result rows.
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However, if your query spans multiple tables, it's possible to get duplicate
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results when a ``QuerySet`` is evaluated. That's when you'd use ``distinct()``.
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``values(*fields)``
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~~~~~~~~~~~~~~~~~~~
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Returns a ``ValuesQuerySet`` -- a ``QuerySet`` that evaluates to a list of
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dictionaries instead of model-instance objects.
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Each of those dictionaries represents an object, with the keys corresponding to
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the attribute names of model objects.
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This example compares the dictionaries of ``values()`` with the normal model
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objects::
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# This list contains a Blog object.
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>>> Blog.objects.filter(name__startswith='Beatles')
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[Beatles Blog]
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# This list contains a dictionary.
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>>> Blog.objects.filter(name__startswith='Beatles').values()
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[{'id': 1, 'name': 'Beatles Blog', 'tagline': 'All the latest Beatles news.'}]
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``values()`` takes optional positional arguments, ``*fields``, which specify
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field names to which the ``SELECT`` should be limited. If you specify the
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fields, each dictionary will contain only the field keys/values for the fields
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you specify. If you don't specify the fields, each dictionary will contain a
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key and value for every field in the database table.
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Example::
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>>> Blog.objects.values()
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[{'id': 1, 'name': 'Beatles Blog', 'tagline': 'All the latest Beatles news.'}],
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>>> Blog.objects.values('id', 'name')
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[{'id': 1, 'name': 'Beatles Blog'}]
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A ``ValuesQuerySet`` is useful when you know you're only going to need values
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from a small number of the available fields and you won't need the
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functionality of a model instance object. It's more efficient to select only
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the fields you need to use.
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Finally, note a ``ValuesQuerySet`` is a subclass of ``QuerySet``, so it has all
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methods of ``QuerySet``. You can call ``filter()`` on it, or ``order_by()``, or
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whatever. Yes, that means these two calls are identical::
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Blog.objects.values().order_by('id')
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Blog.objects.order_by('id').values()
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The people who made Django prefer to put all the SQL-affecting methods first,
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followed (optionally) by any output-affecting methods (such as ``values()``),
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but it doesn't really matter. This is your chance to really flaunt your
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individualism.
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``dates(field, kind, order='ASC')``
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Returns a ``DateQuerySet`` -- a ``QuerySet`` that evaluates to a list of
|
|
``datetime.datetime`` objects representing all available dates of a particular
|
|
kind within the contents of the ``QuerySet``.
|
|
|
|
``field`` should be the name of a ``DateField`` or ``DateTimeField`` of your
|
|
model.
|
|
|
|
``kind`` should be either ``"year"``, ``"month"`` or ``"day"``. Each
|
|
``datetime.datetime`` object in the result list is "truncated" to the given
|
|
``type``.
|
|
|
|
* ``"year"`` returns a list of all distinct year values for the field.
|
|
* ``"month"`` returns a list of all distinct year/month values for the field.
|
|
* ``"day"`` returns a list of all distinct year/month/day values for the field.
|
|
|
|
``order``, which defaults to ``'ASC'``, should be either ``'ASC'`` or
|
|
``'DESC'``. This specifies how to order the results.
|
|
|
|
Examples::
|
|
|
|
>>> Entry.objects.dates('pub_date', 'year')
|
|
[datetime.datetime(2005, 1, 1)]
|
|
>>> Entry.objects.dates('pub_date', 'month')
|
|
[datetime.datetime(2005, 2, 1), datetime.datetime(2005, 3, 1)]
|
|
>>> Entry.objects.dates('pub_date', 'day')
|
|
[datetime.datetime(2005, 2, 20), datetime.datetime(2005, 3, 20)]
|
|
>>> Entry.objects.dates('pub_date', 'day', order='DESC')
|
|
[datetime.datetime(2005, 3, 20), datetime.datetime(2005, 2, 20)]
|
|
>>> Entry.objects.filter(headline__contains='Lennon').dates('pub_date', 'day')
|
|
[datetime.datetime(2005, 3, 20)]
|
|
|
|
``none()``
|
|
~~~~~~~~~~
|
|
|
|
**New in Django development version**
|
|
|
|
Returns an ``EmptyQuerySet`` -- a ``QuerySet`` that always evaluates to
|
|
an empty list. This can be used in cases where you know that you should
|
|
return an empty result set and your caller is expecting a ``QuerySet``
|
|
object (instead of returning an empty list, for example.)
|
|
|
|
Examples::
|
|
|
|
>>> Entry.objects.none()
|
|
[]
|
|
|
|
``select_related()``
|
|
~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Returns a ``QuerySet`` that will automatically "follow" foreign-key
|
|
relationships, selecting that additional related-object data when it executes
|
|
its query. This is a performance booster which results in (sometimes much)
|
|
larger queries but means later use of foreign-key relationships won't require
|
|
database queries.
|
|
|
|
The following examples illustrate the difference between plain lookups and
|
|
``select_related()`` lookups. Here's standard lookup::
|
|
|
|
# Hits the database.
|
|
e = Entry.objects.get(id=5)
|
|
|
|
# Hits the database again to get the related Blog object.
|
|
b = e.blog
|
|
|
|
And here's ``select_related`` lookup::
|
|
|
|
# Hits the database.
|
|
e = Entry.objects.select_related().get(id=5)
|
|
|
|
# Doesn't hit the database, because e.blog has been prepopulated
|
|
# in the previous query.
|
|
b = e.blog
|
|
|
|
``select_related()`` follows foreign keys as far as possible. If you have the
|
|
following models::
|
|
|
|
class City(models.Model):
|
|
# ...
|
|
|
|
class Person(models.Model):
|
|
# ...
|
|
hometown = models.ForeignKey(City)
|
|
|
|
class Book(models.Model):
|
|
# ...
|
|
author = models.ForeignKey(Person)
|
|
|
|
...then a call to ``Book.objects.select_related().get(id=4)`` will cache the
|
|
related ``Person`` *and* the related ``City``::
|
|
|
|
b = Book.objects.select_related().get(id=4)
|
|
p = b.author # Doesn't hit the database.
|
|
c = p.hometown # Doesn't hit the database.
|
|
|
|
sv = Book.objects.get(id=4) # No select_related() in this example.
|
|
p = b.author # Hits the database.
|
|
c = p.hometown # Hits the database.
|
|
|
|
Note that ``select_related()`` does not follow foreign keys that have
|
|
``null=True``.
|
|
|
|
Usually, using ``select_related()`` can vastly improve performance because your
|
|
app can avoid many database calls. However, in situations with deeply nested
|
|
sets of relationships ``select_related()`` can sometimes end up following "too
|
|
many" relations, and can generate queries so large that they end up being slow.
|
|
|
|
In these situations, you can use the ``depth`` argument to ``select_related()``
|
|
to control how many "levels" of relations ``select_related()`` will actually
|
|
follow::
|
|
|
|
b = Book.objects.select_related(depth=1).get(id=4)
|
|
p = b.author # Doesn't hit the database.
|
|
c = p.hometown # Requires a database call.
|
|
|
|
The ``depth`` argument is new in the Django development version.
|
|
|
|
``extra(select=None, where=None, params=None, tables=None)``
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Sometimes, the Django query syntax by itself can't easily express a complex
|
|
``WHERE`` clause. For these edge cases, Django provides the ``extra()``
|
|
``QuerySet`` modifier -- a hook for injecting specific clauses into the SQL
|
|
generated by a ``QuerySet``.
|
|
|
|
By definition, these extra lookups may not be portable to different database
|
|
engines (because you're explicitly writing SQL code) and violate the DRY
|
|
principle, so you should avoid them if possible.
|
|
|
|
Specify one or more of ``params``, ``select``, ``where`` or ``tables``. None
|
|
of the arguments is required, but you should use at least one of them.
|
|
|
|
``select``
|
|
The ``select`` argument lets you put extra fields in the ``SELECT`` clause.
|
|
It should be a dictionary mapping attribute names to SQL clauses to use to
|
|
calculate that attribute.
|
|
|
|
Example::
|
|
|
|
Entry.objects.extra(select={'is_recent': "pub_date > '2006-01-01'"})
|
|
|
|
As a result, each ``Entry`` object will have an extra attribute,
|
|
``is_recent``, a boolean representing whether the entry's ``pub_date`` is
|
|
greater than Jan. 1, 2006.
|
|
|
|
Django inserts the given SQL snippet directly into the ``SELECT``
|
|
statement, so the resulting SQL of the above example would be::
|
|
|
|
SELECT blog_entry.*, (pub_date > '2006-01-01')
|
|
FROM blog_entry;
|
|
|
|
|
|
The next example is more advanced; it does a subquery to give each
|
|
resulting ``Blog`` object an ``entry_count`` attribute, an integer count
|
|
of associated ``Entry`` objects::
|
|
|
|
Blog.objects.extra(
|
|
select={
|
|
'entry_count': 'SELECT COUNT(*) FROM blog_entry WHERE blog_entry.blog_id = blog_blog.id'
|
|
},
|
|
)
|
|
|
|
(In this particular case, we're exploiting the fact that the query will
|
|
already contain the ``blog_blog`` table in its ``FROM`` clause.)
|
|
|
|
The resulting SQL of the above example would be::
|
|
|
|
SELECT blog_blog.*, (SELECT COUNT(*) FROM blog_entry WHERE blog_entry.blog_id = blog_blog.id)
|
|
FROM blog_blog;
|
|
|
|
Note that the parenthesis required by most database engines around
|
|
subqueries are not required in Django's ``select`` clauses. Also note that
|
|
some database backends, such as some MySQL versions, don't support
|
|
subqueries.
|
|
|
|
``where`` / ``tables``
|
|
You can define explicit SQL ``WHERE`` clauses -- perhaps to perform
|
|
non-explicit joins -- by using ``where``. You can manually add tables to
|
|
the SQL ``FROM`` clause by using ``tables``.
|
|
|
|
``where`` and ``tables`` both take a list of strings. All ``where``
|
|
parameters are "AND"ed to any other search criteria.
|
|
|
|
Example::
|
|
|
|
Entry.objects.extra(where=['id IN (3, 4, 5, 20)'])
|
|
|
|
...translates (roughly) into the following SQL::
|
|
|
|
SELECT * FROM blog_entry WHERE id IN (3, 4, 5, 20);
|
|
|
|
``params``
|
|
The ``select`` and ``where`` parameters described above may use standard
|
|
Python database string placeholders -- ``'%s'`` to indicate parameters the
|
|
database engine should automatically quote. The ``params`` argument is a
|
|
list of any extra parameters to be substituted.
|
|
|
|
Example::
|
|
|
|
Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
|
|
|
|
Always use ``params`` instead of embedding values directly into ``select``
|
|
or ``where`` because ``params`` will ensure values are quoted correctly
|
|
according to your particular backend. (For example, quotes will be escaped
|
|
correctly.)
|
|
|
|
Bad::
|
|
|
|
Entry.objects.extra(where=["headline='Lennon'"])
|
|
|
|
Good::
|
|
|
|
Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
|
|
|
|
QuerySet methods that do not return QuerySets
|
|
---------------------------------------------
|
|
|
|
The following ``QuerySet`` methods evaluate the ``QuerySet`` and return
|
|
something *other than* a ``QuerySet``.
|
|
|
|
These methods do not use a cache (see _`Caching and QuerySets` below). Rather,
|
|
they query the database each time they're called.
|
|
|
|
``get(**kwargs)``
|
|
~~~~~~~~~~~~~~~~~
|
|
|
|
Returns the object matching the given lookup parameters, which should be in
|
|
the format described in `Field lookups`_.
|
|
|
|
``get()`` raises ``AssertionError`` if more than one object was found.
|
|
|
|
``get()`` raises a ``DoesNotExist`` exception if an object wasn't found for the
|
|
given parameters. The ``DoesNotExist`` exception is an attribute of the model
|
|
class. Example::
|
|
|
|
Entry.objects.get(id='foo') # raises Entry.DoesNotExist
|
|
|
|
The ``DoesNotExist`` exception inherits from
|
|
``django.core.exceptions.ObjectDoesNotExist``, so you can target multiple
|
|
``DoesNotExist`` exceptions. Example::
|
|
|
|
from django.core.exceptions import ObjectDoesNotExist
|
|
try:
|
|
e = Entry.objects.get(id=3)
|
|
b = Blog.objects.get(id=1)
|
|
except ObjectDoesNotExist:
|
|
print "Either the entry or blog doesn't exist."
|
|
|
|
``create(**kwargs)``
|
|
~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A convenience method for creating an object and saving it all in one step. Thus::
|
|
|
|
p = Person.objects.create(first_name="Bruce", last_name="Springsteen")
|
|
|
|
and::
|
|
|
|
p = Person(first_name="Bruce", last_name="Springsteen")
|
|
p.save()
|
|
|
|
are equivalent.
|
|
|
|
``get_or_create(**kwargs)``
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A convenience method for looking up an object with the given kwargs, creating
|
|
one if necessary.
|
|
|
|
Returns a tuple of ``(object, created)``, where ``object`` is the retrieved or
|
|
created object and ``created`` is a boolean specifying whether a new object was
|
|
created.
|
|
|
|
This is meant as a shortcut to boilerplatish code and is mostly useful for
|
|
data-import scripts. For example::
|
|
|
|
try:
|
|
obj = Person.objects.get(first_name='John', last_name='Lennon')
|
|
except Person.DoesNotExist:
|
|
obj = Person(first_name='John', last_name='Lennon', birthday=date(1940, 10, 9))
|
|
obj.save()
|
|
|
|
This pattern gets quite unwieldy as the number of fields in a model goes up.
|
|
The above example can be rewritten using ``get_or_create()`` like so::
|
|
|
|
obj, created = Person.objects.get_or_create(first_name='John', last_name='Lennon',
|
|
defaults={'birthday': date(1940, 10, 9)})
|
|
|
|
Any keyword arguments passed to ``get_or_create()`` -- *except* an optional one
|
|
called ``defaults`` -- will be used in a ``get()`` call. If an object is found,
|
|
``get_or_create()`` returns a tuple of that object and ``False``. If an object
|
|
is *not* found, ``get_or_create()`` will instantiate and save a new object,
|
|
returning a tuple of the new object and ``True``. The new object will be
|
|
created according to this algorithm::
|
|
|
|
defaults = kwargs.pop('defaults', {})
|
|
params = dict([(k, v) for k, v in kwargs.items() if '__' not in k])
|
|
params.update(defaults)
|
|
obj = self.model(**params)
|
|
obj.save()
|
|
|
|
In English, that means start with any non-``'defaults'`` keyword argument that
|
|
doesn't contain a double underscore (which would indicate a non-exact lookup).
|
|
Then add the contents of ``defaults``, overriding any keys if necessary, and
|
|
use the result as the keyword arguments to the model class.
|
|
|
|
If you have a field named ``defaults`` and want to use it as an exact lookup in
|
|
``get_or_create()``, just use ``'defaults__exact'``, like so::
|
|
|
|
Foo.objects.get_or_create(defaults__exact='bar', defaults={'defaults': 'baz'})
|
|
|
|
Finally, a word on using ``get_or_create()`` in Django views. As mentioned
|
|
earlier, ``get_or_create()`` is mostly useful in scripts that need to parse
|
|
data and create new records if existing ones aren't available. But if you need
|
|
to use ``get_or_create()`` in a view, please make sure to use it only in
|
|
``POST`` requests unless you have a good reason not to. ``GET`` requests
|
|
shouldn't have any effect on data; use ``POST`` whenever a request to a page
|
|
has a side effect on your data. For more, see `Safe methods`_ in the HTTP spec.
|
|
|
|
.. _Safe methods: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.1
|
|
|
|
``count()``
|
|
~~~~~~~~~~~
|
|
|
|
Returns an integer representing the number of objects in the database matching
|
|
the ``QuerySet``. ``count()`` never raises exceptions.
|
|
|
|
Example::
|
|
|
|
# Returns the total number of entries in the database.
|
|
Entry.objects.count()
|
|
|
|
# Returns the number of entries whose headline contains 'Lennon'
|
|
Entry.objects.filter(headline__contains='Lennon').count()
|
|
|
|
``count()`` performs a ``SELECT COUNT(*)`` behind the scenes, so you should
|
|
always use ``count()`` rather than loading all of the record into Python
|
|
objects and calling ``len()`` on the result.
|
|
|
|
Depending on which database you're using (e.g. PostgreSQL vs. MySQL),
|
|
``count()`` may return a long integer instead of a normal Python integer. This
|
|
is an underlying implementation quirk that shouldn't pose any real-world
|
|
problems.
|
|
|
|
``in_bulk(id_list)``
|
|
~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Takes a list of primary-key values and returns a dictionary mapping each
|
|
primary-key value to an instance of the object with the given ID.
|
|
|
|
Example::
|
|
|
|
>>> Blog.objects.in_bulk([1])
|
|
{1: Beatles Blog}
|
|
>>> Blog.objects.in_bulk([1, 2])
|
|
{1: Beatles Blog, 2: Cheddar Talk}
|
|
>>> Blog.objects.in_bulk([])
|
|
{}
|
|
|
|
If you pass ``in_bulk()`` an empty list, you'll get an empty dictionary.
|
|
|
|
``latest(field_name=None)``
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Returns the latest object in the table, by date, using the ``field_name``
|
|
provided as the date field.
|
|
|
|
This example returns the latest ``Entry`` in the table, according to the
|
|
``pub_date`` field::
|
|
|
|
Entry.objects.latest('pub_date')
|
|
|
|
If your model's ``Meta`` specifies ``get_latest_by``, you can leave off the
|
|
``field_name`` argument to ``latest()``. Django will use the field specified in
|
|
``get_latest_by`` by default.
|
|
|
|
Like ``get()``, ``latest()`` raises ``DoesNotExist`` if an object doesn't
|
|
exist with the given parameters.
|
|
|
|
Note ``latest()`` exists purely for convenience and readability.
|
|
|
|
Field lookups
|
|
-------------
|
|
|
|
Field lookups are how you specify the meat of an SQL ``WHERE`` clause. They're
|
|
specified as keyword arguments to the ``QuerySet`` methods ``filter()``,
|
|
``exclude()`` and ``get()``.
|
|
|
|
Basic lookups keyword arguments take the form ``field__lookuptype=value``.
|
|
(That's a double-underscore). For example::
|
|
|
|
Entry.objects.filter(pub_date__lte='2006-01-01')
|
|
|
|
translates (roughly) into the following SQL::
|
|
|
|
SELECT * FROM blog_entry WHERE pub_date <= '2006-01-01';
|
|
|
|
.. admonition:: How this is possible
|
|
|
|
Python has the ability to define functions that accept arbitrary name-value
|
|
arguments whose names and values are evaluated at runtime. For more
|
|
information, see `Keyword Arguments`_ in the official Python tutorial.
|
|
|
|
.. _`Keyword Arguments`: http://docs.python.org/tut/node6.html#SECTION006720000000000000000
|
|
|
|
If you pass an invalid keyword argument, a lookup function will raise
|
|
``TypeError``.
|
|
|
|
The database API supports the following lookup types:
|
|
|
|
exact
|
|
~~~~~
|
|
|
|
Exact match. If the value provided for comparison is ``None``, it will
|
|
be interpreted as an SQL ``NULL`` (See isnull_ for more details).
|
|
|
|
Examples::
|
|
|
|
Entry.objects.get(id__exact=14)
|
|
Entry.objects.get(id__exact=None)
|
|
|
|
SQL equivalents::
|
|
|
|
SELECT ... WHERE id = 14;
|
|
SELECT ... WHERE id = NULL;
|
|
|
|
iexact
|
|
~~~~~~
|
|
|
|
Case-insensitive exact match.
|
|
|
|
Example::
|
|
|
|
Blog.objects.get(name__iexact='beatles blog')
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE name ILIKE 'beatles blog';
|
|
|
|
Note this will match ``'Beatles Blog'``, ``'beatles blog'``,
|
|
``'BeAtLes BLoG'``, etc.
|
|
|
|
contains
|
|
~~~~~~~~
|
|
|
|
Case-sensitive containment test.
|
|
|
|
Example::
|
|
|
|
Entry.objects.get(headline__contains='Lennon')
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE headline LIKE '%Lennon%';
|
|
|
|
Note this will match the headline ``'Today Lennon honored'`` but not
|
|
``'today lennon honored'``.
|
|
|
|
SQLite doesn't support case-sensitive ``LIKE`` statements; ``contains`` acts
|
|
like ``icontains`` for SQLite.
|
|
|
|
icontains
|
|
~~~~~~~~~
|
|
|
|
Case-insensitive containment test.
|
|
|
|
Example::
|
|
|
|
Entry.objects.get(headline__icontains='Lennon')
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE headline ILIKE '%Lennon%';
|
|
|
|
gt
|
|
~~
|
|
|
|
Greater than.
|
|
|
|
Example::
|
|
|
|
Entry.objects.filter(id__gt=4)
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE id > 4;
|
|
|
|
gte
|
|
~~~
|
|
|
|
Greater than or equal to.
|
|
|
|
lt
|
|
~~
|
|
|
|
Less than.
|
|
|
|
lte
|
|
~~~
|
|
|
|
Less than or equal to.
|
|
|
|
in
|
|
~~
|
|
|
|
In a given list.
|
|
|
|
Example::
|
|
|
|
Entry.objects.filter(id__in=[1, 3, 4])
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE id IN (1, 3, 4);
|
|
|
|
startswith
|
|
~~~~~~~~~~
|
|
|
|
Case-sensitive starts-with.
|
|
|
|
Example::
|
|
|
|
Entry.objects.filter(headline__startswith='Will')
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE headline LIKE 'Will%';
|
|
|
|
SQLite doesn't support case-sensitive ``LIKE`` statements; ``startswith`` acts
|
|
like ``istartswith`` for SQLite.
|
|
|
|
istartswith
|
|
~~~~~~~~~~~
|
|
|
|
Case-insensitive starts-with.
|
|
|
|
Example::
|
|
|
|
Entry.objects.filter(headline__istartswith='will')
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE headline ILIKE 'Will%';
|
|
|
|
endswith
|
|
~~~~~~~~
|
|
|
|
Case-sensitive ends-with.
|
|
|
|
Example::
|
|
|
|
Entry.objects.filter(headline__endswith='cats')
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE headline LIKE '%cats';
|
|
|
|
SQLite doesn't support case-sensitive ``LIKE`` statements; ``endswith`` acts
|
|
like ``iendswith`` for SQLite.
|
|
|
|
iendswith
|
|
~~~~~~~~~
|
|
|
|
Case-insensitive ends-with.
|
|
|
|
Example::
|
|
|
|
Entry.objects.filter(headline__iendswith='will')
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE headline ILIKE '%will'
|
|
|
|
range
|
|
~~~~~
|
|
|
|
Range test (inclusive).
|
|
|
|
Example::
|
|
|
|
start_date = datetime.date(2005, 1, 1)
|
|
end_date = datetime.date(2005, 3, 31)
|
|
Entry.objects.filter(pub_date__range=(start_date, end_date))
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE pub_date BETWEEN '2005-01-01' and '2005-03-31';
|
|
|
|
You can use ``range`` anywhere you can use ``BETWEEN`` in SQL -- for dates,
|
|
numbers and even characters.
|
|
|
|
year
|
|
~~~~
|
|
|
|
For date/datetime fields, exact year match. Takes a four-digit year.
|
|
|
|
Example::
|
|
|
|
Entry.objects.filter(pub_date__year=2005)
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE EXTRACT('year' FROM pub_date) = '2005';
|
|
|
|
(The exact SQL syntax varies for each database engine.)
|
|
|
|
month
|
|
~~~~~
|
|
|
|
For date/datetime fields, exact month match. Takes an integer 1 (January)
|
|
through 12 (December).
|
|
|
|
Example::
|
|
|
|
Entry.objects.filter(pub_date__month=12)
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE EXTRACT('month' FROM pub_date) = '12';
|
|
|
|
(The exact SQL syntax varies for each database engine.)
|
|
|
|
day
|
|
~~~
|
|
|
|
For date/datetime fields, exact day match.
|
|
|
|
Example::
|
|
|
|
Entry.objects.filter(pub_date__day=3)
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE EXTRACT('day' FROM pub_date) = '3';
|
|
|
|
(The exact SQL syntax varies for each database engine.)
|
|
|
|
Note this will match any record with a pub_date on the third day of the month,
|
|
such as January 3, July 3, etc.
|
|
|
|
isnull
|
|
~~~~~~
|
|
|
|
Takes either ``True`` or ``False``, which correspond to SQL queries of
|
|
``IS NULL`` and ``IS NOT NULL``, respectively.
|
|
|
|
Example::
|
|
|
|
Entry.objects.filter(pub_date__isnull=True)
|
|
|
|
SQL equivalent::
|
|
|
|
SELECT ... WHERE pub_date IS NULL;
|
|
|
|
.. admonition:: ``__isnull=True`` vs ``__exact=None``
|
|
|
|
There is an important difference between ``__isnull=True`` and
|
|
``__exact=None``. ``__exact=None`` will *always* return an empty result
|
|
set, because SQL requires that no value is equal to ``NULL``.
|
|
``__isnull`` determines if the field is currently holding the value
|
|
of ``NULL`` without performing a comparison.
|
|
|
|
search
|
|
~~~~~~
|
|
|
|
A boolean full-text search, taking advantage of full-text indexing. This is
|
|
like ``contains`` but is significantly faster due to full-text indexing.
|
|
|
|
Note this is only available in MySQL and requires direct manipulation of the
|
|
database to add the full-text index.
|
|
|
|
Default lookups are exact
|
|
-------------------------
|
|
|
|
If you don't provide a lookup type -- that is, if your keyword argument doesn't
|
|
contain a double underscore -- the lookup type is assumed to be ``exact``.
|
|
|
|
For example, the following two statements are equivalent::
|
|
|
|
Blog.objects.get(id__exact=14) # Explicit form
|
|
Blog.objects.get(id=14) # __exact is implied
|
|
|
|
This is for convenience, because ``exact`` lookups are the common case.
|
|
|
|
The pk lookup shortcut
|
|
----------------------
|
|
|
|
For convenience, Django provides a ``pk`` lookup type, which stands for
|
|
"primary_key".
|
|
|
|
In the example ``Blog`` model, the primary key is the ``id`` field, so these
|
|
three statements are equivalent::
|
|
|
|
Blog.objects.get(id__exact=14) # Explicit form
|
|
Blog.objects.get(id=14) # __exact is implied
|
|
Blog.objects.get(pk=14) # pk implies id__exact
|
|
|
|
The use of ``pk`` isn't limited to ``__exact`` queries -- any query term
|
|
can be combined with ``pk`` to perform a query on the primary key of a model::
|
|
|
|
# Get blogs entries with id 1, 4 and 7
|
|
Blog.objects.filter(pk__in=[1,4,7])
|
|
# Get all blog entries with id > 14
|
|
Blog.objects.filter(pk__gt=14)
|
|
|
|
``pk`` lookups also work across joins. For example, these three statements are
|
|
equivalent::
|
|
|
|
Entry.objects.filter(blog__id__exact=3) # Explicit form
|
|
Entry.objects.filter(blog__id=3) # __exact is implied
|
|
Entry.objects.filter(blog__pk=3) # __pk implies __id__exact
|
|
|
|
Lookups that span relationships
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Django offers a powerful and intuitive way to "follow" relationships in
|
|
lookups, taking care of the SQL ``JOIN``\s for you automatically, behind the
|
|
scenes. To span a relationship, just use the field name of related fields
|
|
across models, separated by double underscores, until you get to the field you
|
|
want.
|
|
|
|
This example retrieves all ``Entry`` objects with a ``Blog`` whose ``name``
|
|
is ``'Beatles Blog'``::
|
|
|
|
Entry.objects.filter(blog__name__exact='Beatles Blog')
|
|
|
|
This spanning can be as deep as you'd like.
|
|
|
|
It works backwards, too. To refer to a "reverse" relationship, just use the
|
|
lowercase name of the model.
|
|
|
|
This example retrieves all ``Blog`` objects which have at least one ``Entry``
|
|
whose ``headline`` contains ``'Lennon'``::
|
|
|
|
Blog.objects.filter(entry__headline__contains='Lennon')
|
|
|
|
Escaping parenthesis and underscores in LIKE statements
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The field lookups that equate to ``LIKE`` SQL statements (``iexact``,
|
|
``contains``, ``icontains``, ``startswith``, ``istartswith``, ``endswith``
|
|
and ``iendswith``) will automatically escape the two special characters used in
|
|
``LIKE`` statements -- the percent sign and the underscore. (In a ``LIKE``
|
|
statement, the percent sign signifies a multiple-character wildcard and the
|
|
underscore signifies a single-character wildcard.)
|
|
|
|
This means things should work intuitively, so the abstraction doesn't leak.
|
|
For example, to retrieve all the entries that contain a percent sign, just use
|
|
the percent sign as any other character::
|
|
|
|
Entry.objects.filter(headline__contains='%')
|
|
|
|
Django takes care of the quoting for you; the resulting SQL will look something
|
|
like this::
|
|
|
|
SELECT ... WHERE headline LIKE '%\%%';
|
|
|
|
Same goes for underscores. Both percentage signs and underscores are handled
|
|
for you transparently.
|
|
|
|
Caching and QuerySets
|
|
---------------------
|
|
|
|
Each ``QuerySet`` contains a cache, to minimize database access. It's important
|
|
to understand how it works, in order to write the most efficient code.
|
|
|
|
In a newly created ``QuerySet``, the cache is empty. The first time a
|
|
``QuerySet`` is evaluated -- and, hence, a database query happens -- Django
|
|
saves the query results in the ``QuerySet``'s cache and returns the results
|
|
that have been explicitly requested (e.g., the next element, if the
|
|
``QuerySet`` is being iterated over). Subsequent evaluations of the
|
|
``QuerySet`` reuse the cached results.
|
|
|
|
Keep this caching behavior in mind, because it may bite you if you don't use
|
|
your ``QuerySet``\s correctly. For example, the following will create two
|
|
``QuerySet``\s, evaluate them, and throw them away::
|
|
|
|
print [e.headline for e in Entry.objects.all()]
|
|
print [e.pub_date for e in Entry.objects.all()]
|
|
|
|
That means the same database query will be executed twice, effectively doubling
|
|
your database load. Also, there's a possibility the two lists may not include
|
|
the same database records, because an ``Entry`` may have been added or deleted
|
|
in the split second between the two requests.
|
|
|
|
To avoid this problem, simply save the ``QuerySet`` and reuse it::
|
|
|
|
queryset = Poll.objects.all()
|
|
print [p.headline for p in queryset] # Evaluate the query set.
|
|
print [p.pub_date for p in queryset] # Re-use the cache from the evaluation.
|
|
|
|
Comparing objects
|
|
=================
|
|
|
|
To compare two model instances, just use the standard Python comparison operator,
|
|
the double equals sign: ``==``. Behind the scenes, that compares the primary
|
|
key values of two models.
|
|
|
|
Using the ``Entry`` example above, the following two statements are equivalent::
|
|
|
|
some_entry == other_entry
|
|
some_entry.id == other_entry.id
|
|
|
|
If a model's primary key isn't called ``id``, no problem. Comparisons will
|
|
always use the primary key, whatever it's called. For example, if a model's
|
|
primary key field is called ``name``, these two statements are equivalent::
|
|
|
|
some_obj == other_obj
|
|
some_obj.name == other_obj.name
|
|
|
|
Complex lookups with Q objects
|
|
==============================
|
|
|
|
Keyword argument queries -- in ``filter()``, etc. -- are "AND"ed together. If
|
|
you need to execute more complex queries (for example, queries with ``OR``
|
|
statements), you can use ``Q`` objects.
|
|
|
|
A ``Q`` object (``django.db.models.Q``) is an object used to encapsulate a
|
|
collection of keyword arguments. These keyword arguments are specified as in
|
|
"Field lookups" above.
|
|
|
|
For example, this ``Q`` object encapsulates a single ``LIKE`` query::
|
|
|
|
Q(question__startswith='What')
|
|
|
|
``Q`` objects can be combined using the ``&`` and ``|`` operators. When an
|
|
operator is used on two ``Q`` objects, it yields a new ``Q`` object.
|
|
|
|
For example, this statement yields a single ``Q`` object that represents the
|
|
"OR" of two ``"question__startswith"`` queries::
|
|
|
|
Q(question__startswith='Who') | Q(question__startswith='What')
|
|
|
|
This is equivalent to the following SQL ``WHERE`` clause::
|
|
|
|
WHERE question LIKE 'Who%' OR question LIKE 'What%'
|
|
|
|
You can compose statements of arbitrary complexity by combining ``Q`` objects
|
|
with the ``&`` and ``|`` operators. You can also use parenthetical grouping.
|
|
|
|
Each lookup function that takes keyword-arguments (e.g. ``filter()``,
|
|
``exclude()``, ``get()``) can also be passed one or more ``Q`` objects as
|
|
positional (not-named) arguments. If you provide multiple ``Q`` object
|
|
arguments to a lookup function, the arguments will be "AND"ed together. For
|
|
example::
|
|
|
|
Poll.objects.get(
|
|
Q(question__startswith='Who'),
|
|
Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6))
|
|
)
|
|
|
|
... roughly translates into the SQL::
|
|
|
|
SELECT * from polls WHERE question LIKE 'Who%'
|
|
AND (pub_date = '2005-05-02' OR pub_date = '2005-05-06')
|
|
|
|
Lookup functions can mix the use of ``Q`` objects and keyword arguments. All
|
|
arguments provided to a lookup function (be they keyword arguments or ``Q``
|
|
objects) are "AND"ed together. However, if a ``Q`` object is provided, it must
|
|
precede the definition of any keyword arguments. For example::
|
|
|
|
Poll.objects.get(
|
|
Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6)),
|
|
question__startswith='Who')
|
|
|
|
... would be a valid query, equivalent to the previous example; but::
|
|
|
|
# INVALID QUERY
|
|
Poll.objects.get(
|
|
question__startswith='Who',
|
|
Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6)))
|
|
|
|
... would not be valid.
|
|
|
|
See the `OR lookups examples page`_ for more examples.
|
|
|
|
.. _OR lookups examples page: http://www.djangoproject.com/documentation/models/or_lookups/
|
|
|
|
Related objects
|
|
===============
|
|
|
|
When you define a relationship in a model (i.e., a ``ForeignKey``,
|
|
``OneToOneField``, or ``ManyToManyField``), instances of that model will have
|
|
a convenient API to access the related object(s).
|
|
|
|
Using the models at the top of this page, for example, an ``Entry`` object ``e``
|
|
can get its associated ``Blog`` object by accessing the ``blog`` attribute:
|
|
``e.blog``.
|
|
|
|
(Behind the scenes, this functionality is implemented by Python descriptors_.
|
|
This shouldn't really matter to you, but we point it out here for the curious.)
|
|
|
|
Django also creates API accessors for the "other" side of the relationship --
|
|
the link from the related model to the model that defines the relationship.
|
|
For example, a ``Blog`` object ``b`` has access to a list of all related
|
|
``Entry`` objects via the ``entry_set`` attribute: ``b.entry_set.all()``.
|
|
|
|
All examples in this section use the sample ``Blog``, ``Author`` and ``Entry``
|
|
models defined at the top of this page.
|
|
|
|
.. _descriptors: http://users.rcn.com/python/download/Descriptor.htm
|
|
|
|
One-to-many relationships
|
|
-------------------------
|
|
|
|
Forward
|
|
~~~~~~~
|
|
|
|
If a model has a ``ForeignKey``, instances of that model will have access to
|
|
the related (foreign) object via a simple attribute of the model.
|
|
|
|
Example::
|
|
|
|
e = Entry.objects.get(id=2)
|
|
e.blog # Returns the related Blog object.
|
|
|
|
You can get and set via a foreign-key attribute. As you may expect, changes to
|
|
the foreign key aren't saved to the database until you call ``save()``.
|
|
Example::
|
|
|
|
e = Entry.objects.get(id=2)
|
|
e.blog = some_blog
|
|
e.save()
|
|
|
|
If a ``ForeignKey`` field has ``null=True`` set (i.e., it allows ``NULL``
|
|
values), you can assign ``None`` to it. Example::
|
|
|
|
e = Entry.objects.get(id=2)
|
|
e.blog = None
|
|
e.save() # "UPDATE blog_entry SET blog_id = NULL ...;"
|
|
|
|
Forward access to one-to-many relationships is cached the first time the
|
|
related object is accessed. Subsequent accesses to the foreign key on the same
|
|
object instance are cached. Example::
|
|
|
|
e = Entry.objects.get(id=2)
|
|
print e.blog # Hits the database to retrieve the associated Blog.
|
|
print e.blog # Doesn't hit the database; uses cached version.
|
|
|
|
Note that the ``select_related()`` ``QuerySet`` method recursively prepopulates
|
|
the cache of all one-to-many relationships ahead of time. Example::
|
|
|
|
e = Entry.objects.select_related().get(id=2)
|
|
print e.blog # Doesn't hit the database; uses cached version.
|
|
print e.blog # Doesn't hit the database; uses cached version.
|
|
|
|
``select_related()`` is documented in the "QuerySet methods that return new
|
|
QuerySets" section above.
|
|
|
|
Backward
|
|
~~~~~~~~
|
|
|
|
If a model has a ``ForeignKey``, instances of the foreign-key model will have
|
|
access to a ``Manager`` that returns all instances of the first model. By
|
|
default, this ``Manager`` is named ``FOO_set``, where ``FOO`` is the source
|
|
model name, lowercased. This ``Manager`` returns ``QuerySets``, which can be
|
|
filtered and manipulated as described in the "Retrieving objects" section
|
|
above.
|
|
|
|
Example::
|
|
|
|
b = Blog.objects.get(id=1)
|
|
b.entry_set.all() # Returns all Entry objects related to Blog.
|
|
|
|
# b.entry_set is a Manager that returns QuerySets.
|
|
b.entry_set.filter(headline__contains='Lennon')
|
|
b.entry_set.count()
|
|
|
|
You can override the ``FOO_set`` name by setting the ``related_name``
|
|
parameter in the ``ForeignKey()`` definition. For example, if the ``Entry``
|
|
model was altered to ``blog = ForeignKey(Blog, related_name='entries')``, the
|
|
above example code would look like this::
|
|
|
|
b = Blog.objects.get(id=1)
|
|
b.entries.all() # Returns all Entry objects related to Blog.
|
|
|
|
# b.entries is a Manager that returns QuerySets.
|
|
b.entries.filter(headline__contains='Lennon')
|
|
b.entries.count()
|
|
|
|
You cannot access a reverse ``ForeignKey`` ``Manager`` from the class; it must
|
|
be accessed from an instance. Example::
|
|
|
|
Blog.entry_set # Raises AttributeError: "Manager must be accessed via instance".
|
|
|
|
In addition to the ``QuerySet`` methods defined in "Retrieving objects" above,
|
|
the ``ForeignKey`` ``Manager`` has these additional methods:
|
|
|
|
* ``add(obj1, obj2, ...)``: Adds the specified model objects to the related
|
|
object set.
|
|
|
|
Example::
|
|
|
|
b = Blog.objects.get(id=1)
|
|
e = Entry.objects.get(id=234)
|
|
b.entry_set.add(e) # Associates Entry e with Blog b.
|
|
|
|
* ``create(**kwargs)``: Creates a new object, saves it and puts it in the
|
|
related object set. Returns the newly created object.
|
|
|
|
Example::
|
|
|
|
b = Blog.objects.get(id=1)
|
|
e = b.entry_set.create(headline='Hello', body_text='Hi', pub_date=datetime.date(2005, 1, 1))
|
|
# No need to call e.save() at this point -- it's already been saved.
|
|
|
|
This is equivalent to (but much simpler than)::
|
|
|
|
b = Blog.objects.get(id=1)
|
|
e = Entry(blog=b, headline='Hello', body_text='Hi', pub_date=datetime.date(2005, 1, 1))
|
|
e.save()
|
|
|
|
Note that there's no need to specify the keyword argument of the model
|
|
that defines the relationship. In the above example, we don't pass the
|
|
parameter ``blog`` to ``create()``. Django figures out that the new
|
|
``Entry`` object's ``blog`` field should be set to ``b``.
|
|
|
|
* ``remove(obj1, obj2, ...)``: Removes the specified model objects from the
|
|
related object set.
|
|
|
|
Example::
|
|
|
|
b = Blog.objects.get(id=1)
|
|
e = Entry.objects.get(id=234)
|
|
b.entry_set.remove(e) # Disassociates Entry e from Blog b.
|
|
|
|
In order to prevent database inconsistency, this method only exists on
|
|
``ForeignKey`` objects where ``null=True``. If the related field can't be
|
|
set to ``None`` (``NULL``), then an object can't be removed from a
|
|
relation without being added to another. In the above example, removing
|
|
``e`` from ``b.entry_set()`` is equivalent to doing ``e.blog = None``,
|
|
and because the ``blog`` ``ForeignKey`` doesn't have ``null=True``, this
|
|
is invalid.
|
|
|
|
* ``clear()``: Removes all objects from the related object set.
|
|
|
|
Example::
|
|
|
|
b = Blog.objects.get(id=1)
|
|
b.entry_set.clear()
|
|
|
|
Note this doesn't delete the related objects -- it just disassociates
|
|
them.
|
|
|
|
Just like ``remove()``, ``clear()`` is only available on ``ForeignKey``s
|
|
where ``null=True``.
|
|
|
|
To assign the members of a related set in one fell swoop, just assign to it
|
|
from any iterable object. Example::
|
|
|
|
b = Blog.objects.get(id=1)
|
|
b.entry_set = [e1, e2]
|
|
|
|
If the ``clear()`` method is available, any pre-existing objects will be
|
|
removed from the ``entry_set`` before all objects in the iterable (in this
|
|
case, a list) are added to the set. If the ``clear()`` method is *not*
|
|
available, all objects in the iterable will be added without removing any
|
|
existing elements.
|
|
|
|
Each "reverse" operation described in this section has an immediate effect on
|
|
the database. Every addition, creation and deletion is immediately and
|
|
automatically saved to the database.
|
|
|
|
Many-to-many relationships
|
|
--------------------------
|
|
|
|
Both ends of a many-to-many relationship get automatic API access to the other
|
|
end. The API works just as a "backward" one-to-many relationship. See Backward_
|
|
above.
|
|
|
|
The only difference is in the attribute naming: The model that defines the
|
|
``ManyToManyField`` uses the attribute name of that field itself, whereas the
|
|
"reverse" model uses the lowercased model name of the original model, plus
|
|
``'_set'`` (just like reverse one-to-many relationships).
|
|
|
|
An example makes this easier to understand::
|
|
|
|
e = Entry.objects.get(id=3)
|
|
e.authors.all() # Returns all Author objects for this Entry.
|
|
e.authors.count()
|
|
e.authors.filter(name__contains='John')
|
|
|
|
a = Author.objects.get(id=5)
|
|
a.entry_set.all() # Returns all Entry objects for this Author.
|
|
|
|
Like ``ForeignKey``, ``ManyToManyField`` can specify ``related_name``. In the
|
|
above example, if the ``ManyToManyField`` in ``Entry`` had specified
|
|
``related_name='entries'``, then each ``Author`` instance would have an
|
|
``entries`` attribute instead of ``entry_set``.
|
|
|
|
One-to-one relationships
|
|
------------------------
|
|
|
|
The semantics of one-to-one relationships will be changing soon, so we don't
|
|
recommend you use them.
|
|
|
|
How are the backward relationships possible?
|
|
--------------------------------------------
|
|
|
|
Other object-relational mappers require you to define relationships on both
|
|
sides. The Django developers believe this is a violation of the DRY (Don't
|
|
Repeat Yourself) principle, so Django only requires you to define the
|
|
relationship on one end.
|
|
|
|
But how is this possible, given that a model class doesn't know which other
|
|
model classes are related to it until those other model classes are loaded?
|
|
|
|
The answer lies in the ``INSTALLED_APPS`` setting. The first time any model is
|
|
loaded, Django iterates over every model in ``INSTALLED_APPS`` and creates the
|
|
backward relationships in memory as needed. Essentially, one of the functions
|
|
of ``INSTALLED_APPS`` is to tell Django the entire model domain.
|
|
|
|
Queries over related objects
|
|
----------------------------
|
|
|
|
Queries involving related objects follow the same rules as queries involving
|
|
normal value fields. When specifying the the value for a query to match, you
|
|
may use either an object instance itself, or the primary key value for the
|
|
object.
|
|
|
|
For example, if you have a Blog object ``b`` with ``id=5``, the following
|
|
three queries would be identical::
|
|
|
|
Entry.objects.filter(blog=b) # Query using object instance
|
|
Entry.objects.filter(blog=b.id) # Query using id from instance
|
|
Entry.objects.filter(blog=5) # Query using id directly
|
|
|
|
Deleting objects
|
|
================
|
|
|
|
The delete method, conveniently, is named ``delete()``. This method immediately
|
|
deletes the object and has no return value. Example::
|
|
|
|
e.delete()
|
|
|
|
You can also delete objects in bulk. Every ``QuerySet`` has a ``delete()``
|
|
method, which deletes all members of that ``QuerySet``.
|
|
|
|
For example, this deletes all ``Entry`` objects with a ``pub_date`` year of
|
|
2005::
|
|
|
|
Entry.objects.filter(pub_date__year=2005).delete()
|
|
|
|
When Django deletes an object, it emulates the behavior of the SQL
|
|
constraint ``ON DELETE CASCADE`` -- in other words, any objects which
|
|
had foreign keys pointing at the object to be deleted will be deleted
|
|
along with it. For example::
|
|
|
|
b = Blog.objects.get(pk=1)
|
|
# This will delete the Blog and all of its Entry objects.
|
|
b.delete()
|
|
|
|
Note that ``delete()`` is the only ``QuerySet`` method that is not exposed on a
|
|
``Manager`` itself. This is a safety mechanism to prevent you from accidentally
|
|
requesting ``Entry.objects.delete()``, and deleting *all* the entries. If you
|
|
*do* want to delete all the objects, then you have to explicitly request a
|
|
complete query set::
|
|
|
|
Entry.objects.all().delete()
|
|
|
|
Extra instance methods
|
|
======================
|
|
|
|
In addition to ``save()``, ``delete()``, a model object might get any or all
|
|
of the following methods:
|
|
|
|
get_FOO_display()
|
|
-----------------
|
|
|
|
For every field that has ``choices`` set, the object will have a
|
|
``get_FOO_display()`` method, where ``FOO`` is the name of the field. This
|
|
method returns the "human-readable" value of the field. For example, in the
|
|
following model::
|
|
|
|
GENDER_CHOICES = (
|
|
('M', 'Male'),
|
|
('F', 'Female'),
|
|
)
|
|
class Person(models.Model):
|
|
name = models.CharField(maxlength=20)
|
|
gender = models.CharField(maxlength=1, choices=GENDER_CHOICES)
|
|
|
|
...each ``Person`` instance will have a ``get_gender_display()`` method. Example::
|
|
|
|
>>> p = Person(name='John', gender='M')
|
|
>>> p.save()
|
|
>>> p.gender
|
|
'M'
|
|
>>> p.get_gender_display()
|
|
'Male'
|
|
|
|
get_next_by_FOO(\**kwargs) and get_previous_by_FOO(\**kwargs)
|
|
-------------------------------------------------------------
|
|
|
|
For every ``DateField`` and ``DateTimeField`` that does not have ``null=True``,
|
|
the object will have ``get_next_by_FOO()`` and ``get_previous_by_FOO()``
|
|
methods, where ``FOO`` is the name of the field. This returns the next and
|
|
previous object with respect to the date field, raising the appropriate
|
|
``DoesNotExist`` exception when appropriate.
|
|
|
|
Both methods accept optional keyword arguments, which should be in the format
|
|
described in `Field lookups`_ above.
|
|
|
|
Note that in the case of identical date values, these methods will use the ID
|
|
as a fallback check. This guarantees that no records are skipped or duplicated.
|
|
For a full example, see the `lookup API sample model`_.
|
|
|
|
.. _lookup API sample model: http://www.djangoproject.com/documentation/models/lookup/
|
|
|
|
get_FOO_filename()
|
|
------------------
|
|
|
|
For every ``FileField``, the object will have a ``get_FOO_filename()`` method,
|
|
where ``FOO`` is the name of the field. This returns the full filesystem path
|
|
to the file, according to your ``MEDIA_ROOT`` setting.
|
|
|
|
Note that ``ImageField`` is technically a subclass of ``FileField``, so every
|
|
model with an ``ImageField`` will also get this method.
|
|
|
|
get_FOO_url()
|
|
-------------
|
|
|
|
For every ``FileField``, the object will have a ``get_FOO_url()`` method,
|
|
where ``FOO`` is the name of the field. This returns the full URL to the file,
|
|
according to your ``MEDIA_URL`` setting. If the value is blank, this method
|
|
returns an empty string.
|
|
|
|
get_FOO_size()
|
|
--------------
|
|
|
|
For every ``FileField``, the object will have a ``get_FOO_size()`` method,
|
|
where ``FOO`` is the name of the field. This returns the size of the file, in
|
|
bytes. (Behind the scenes, it uses ``os.path.getsize``.)
|
|
|
|
save_FOO_file(filename, raw_contents)
|
|
-------------------------------------
|
|
|
|
For every ``FileField``, the object will have a ``save_FOO_file()`` method,
|
|
where ``FOO`` is the name of the field. This saves the given file to the
|
|
filesystem, using the given filename. If a file with the given filename already
|
|
exists, Django adds an underscore to the end of the filename (but before the
|
|
extension) until the filename is available.
|
|
|
|
get_FOO_height() and get_FOO_width()
|
|
------------------------------------
|
|
|
|
For every ``ImageField``, the object will have ``get_FOO_height()`` and
|
|
``get_FOO_width()`` methods, where ``FOO`` is the name of the field. This
|
|
returns the height (or width) of the image, as an integer, in pixels.
|
|
|
|
Shortcuts
|
|
=========
|
|
|
|
As you develop views, you will discover a number of common idioms in the
|
|
way you use the database API. Django encodes some of these idioms as
|
|
shortcuts that can be used to simplify the process of writing views.
|
|
|
|
get_object_or_404()
|
|
-------------------
|
|
|
|
One common idiom to use ``get()`` and raise ``Http404`` if the
|
|
object doesn't exist. This idiom is captured by ``get_object_or_404()``.
|
|
This function takes a Django model as its first argument and an
|
|
arbitrary number of keyword arguments, which it passes to the manager's
|
|
``get()`` function. It raises ``Http404`` if the object doesn't
|
|
exist. For example::
|
|
|
|
# Get the Entry with a primary key of 3
|
|
e = get_object_or_404(Entry, pk=3)
|
|
|
|
When you provide a model to this shortcut function, the default manager
|
|
is used to execute the underlying ``get()`` query. If you don't want to
|
|
use the default manager, or you want to search a list of related objects,
|
|
you can provide ``get_object_or_404()`` with a manager object, instead.
|
|
For example::
|
|
|
|
# Get the author of blog instance `e` with a name of 'Fred'
|
|
a = get_object_or_404(e.authors, name='Fred')
|
|
|
|
# Use a custom manager 'recent_entries' in the search for an
|
|
# entry with a primary key of 3
|
|
e = get_object_or_404(Entry.recent_entries, pk=3)
|
|
|
|
get_list_or_404()
|
|
-----------------
|
|
|
|
``get_list_or_404`` behaves the same was as ``get_object_or_404()``
|
|
-- except the it uses using ``filter()`` instead of ``get()``. It raises
|
|
``Http404`` if the list is empty.
|
|
|
|
Falling back to raw SQL
|
|
=======================
|
|
|
|
If you find yourself needing to write an SQL query that is too complex for
|
|
Django's database-mapper to handle, you can fall back into raw-SQL statement
|
|
mode.
|
|
|
|
The preferred way to do this is by giving your model custom methods or custom
|
|
manager methods that execute queries. Although there's nothing in Django that
|
|
*requires* database queries to live in the model layer, this approach keeps all
|
|
your data-access logic in one place, which is smart from an code-organization
|
|
standpoint. For instructions, see `Executing custom SQL`_.
|
|
|
|
Finally, it's important to note that the Django database layer is merely an
|
|
interface to your database. You can access your database via other tools,
|
|
programming languages or database frameworks; there's nothing Django-specific
|
|
about your database.
|
|
|
|
.. _Executing custom SQL: ../model_api/#executing-custom-sql
|