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408 lines
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408 lines
16 KiB
Plaintext
.. _ref-models-instances:
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========================
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Model instance reference
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========================
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.. currentmodule:: django.db.models
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This document describes the details of the ``Model`` API. It builds on the
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material presented in the :ref:`model <topics-db-models>` and `database query
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<topics-db-queries>` guides, so you'll probably want to read and understand
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those documents before reading this one.
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Throughout this reference we'll use the :ref:`example weblog models
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<queryset-model-example>` presented in the :ref:`database query guide
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<topics-db-queries>`.
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Creating objects
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================
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To create a new instance of a model, just instantiate it like any other Python class:
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.. class:: Model(**kwargs)
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The keyword arguments to are simply the names of the fields you've defined on
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your model. Note that instantiating a model in no way touches your database; for
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that, you need to ``save()``.
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Saving objects
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==============
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To save an object back to the database, call ``save()``:
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.. method:: Model.save([force_insert=False, force_update=False])
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Of course, there are some subtleties; see the sections below.
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.. versionadded:: 1.0
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The signature of the ``save()`` method has changed from earlier versions
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(``force_insert`` and ``force_update`` have been added). If you are overriding
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these methods, be sure to use the correct signature.
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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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>>> 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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documentation for ``AutoField`` for more details.
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The ``pk`` property
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~~~~~~~~~~~~~~~~~~~
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.. versionadded:: 1.0
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.. attribute:: Model.pk
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Regardless of whether you define a primary key field yourself, or let Django
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supply one for you, each model will have a property called ``pk``. It behaves
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like a normal attribute on the model, but is actually an alias for whichever
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attribute is the primary key field for the model. You can read and set this
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value, just as you would for any other attribute, and it will update the
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correct field in the model.
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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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>>> 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 you're
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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 the
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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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What happens when you save?
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---------------------------
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When you save an object, Django performs the following steps:
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1. **Emit a pre-save signal.** The :ref:`signal <ref-signals>`
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:attr:`django.db.models.signals.pre_save` is sent, allowing any
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functions listening for that signal to take some customized
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action.
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2. **Pre-process the data.** Each field on the object is asked to
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perform any automated data modification that the field may need
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to perform.
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Most fields do *no* pre-processing -- the field data is kept as-is.
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Pre-processing is only used on fields that have special behavior.
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For example, if your model has a ``DateField`` with ``auto_now=True``,
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the pre-save phase will alter the data in the object to ensure that
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the date field contains the current date stamp. (Our documentation
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doesn't yet include a list of all the fields with this "special
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behavior.")
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3. **Prepare the data for the database.** Each field is asked to provide
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its current value in a data type that can be written to the database.
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Most fields require *no* data preparation. Simple data types, such as
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integers and strings, are 'ready to write' as a Python object. However,
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more complex data types often require some modification.
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For example, ``DateFields`` use a Python ``datetime`` object to store
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data. Databases don't store ``datetime`` objects, so the field value
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must be converted into an ISO-compliant date string for insertion
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into the database.
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4. **Insert the data into the database.** The pre-processed, prepared
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data is then composed into an SQL statement for insertion into the
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database.
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5. **Emit a post-save signal.** The signal
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:attr:`django.db.models.signals.post_save` is sent, allowing
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any functions listening for that signal to take some customized
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action.
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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 `Explicitly specifying
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auto-primary-key values`_ above and `Forcing an INSERT or UPDATE`_ below.
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.. _ref-models-force-insert:
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Forcing an INSERT or UPDATE
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. versionadded:: 1.0
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In some rare circumstances, it's necessary to be able to force the ``save()``
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method to perform an SQL ``INSERT`` and not fall back to doing an ``UPDATE``.
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Or vice-versa: update, if possible, but not insert a new row. In these cases
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you can pass the ``force_insert=True`` or ``force_update=True`` parameters to
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the ``save()`` method. Passing both parameters is an error, since you cannot
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both insert *and* update at the same time.
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It should be very rare that you'll need to use these parameters. Django will
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almost always do the right thing and trying to override that will lead to
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errors that are difficult to track down. This feature is for advanced use
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only.
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.. _model-instance-methods:
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Other model instance methods
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============================
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A few object methods have special purposes.
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``__str__``
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-----------
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.. method:: Model.__str__()
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``__str__()`` is a Python "magic method" that defines what should be returned
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if you call ``str()`` on the object. Django uses ``str(obj)`` (or the related
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function, ``unicode(obj)`` -- see below) in a number of places, most notably
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as the value displayed to render an object in the Django admin site and as the
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value inserted into a template when it displays an object. Thus, you should
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always return a nice, human-readable string for the object's ``__str__``.
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Although this isn't required, it's strongly encouraged (see the description of
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``__unicode__``, below, before putting ``__str__`` methods everywhere).
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For example::
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class Person(models.Model):
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first_name = models.CharField(max_length=50)
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last_name = models.CharField(max_length=50)
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def __str__(self):
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# Note use of django.utils.encoding.smart_str() here because
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# first_name and last_name will be unicode strings.
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return smart_str('%s %s' % (self.first_name, self.last_name))
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``__unicode__``
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---------------
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.. method:: Model.__unicode__()
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The ``__unicode__()`` method is called whenever you call ``unicode()`` on an
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object. Since Django's database backends will return Unicode strings in your
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model's attributes, you would normally want to write a ``__unicode__()``
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method for your model. The example in the previous section could be written
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more simply as::
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class Person(models.Model):
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first_name = models.CharField(max_length=50)
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last_name = models.CharField(max_length=50)
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def __unicode__(self):
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return u'%s %s' % (self.first_name, self.last_name)
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If you define a ``__unicode__()`` method on your model and not a ``__str__()``
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method, Django will automatically provide you with a ``__str__()`` that calls
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``__unicode__()`` and then converts the result correctly to a UTF-8 encoded
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string object. This is recommended development practice: define only
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``__unicode__()`` and let Django take care of the conversion to string objects
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when required.
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``get_absolute_url``
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--------------------
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.. method:: Model.get_absolute_url()
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Define a ``get_absolute_url()`` method to tell Django how to calculate the
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URL for an object. For example::
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def get_absolute_url(self):
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return "/people/%i/" % self.id
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Django uses this in its admin interface. If an object defines
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``get_absolute_url()``, the object-editing page will have a "View on site"
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link that will jump you directly to the object's public view, according to
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``get_absolute_url()``.
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Also, a couple of other bits of Django, such as the :ref:`syndication feed
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framework <ref-contrib-syndication>`, use ``get_absolute_url()`` as a
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convenience to reward people who've defined the method.
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It's good practice to use ``get_absolute_url()`` in templates, instead of
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hard-coding your objects' URLs. For example, this template code is bad::
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<a href="/people/{{ object.id }}/">{{ object.name }}</a>
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But this template code is good::
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<a href="{{ object.get_absolute_url }}">{{ object.name }}</a>
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.. note::
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The string you return from ``get_absolute_url()`` must contain only ASCII
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characters (required by the URI spec, `RFC 2396`_) that have been
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URL-encoded, if necessary. Code and templates using ``get_absolute_url()``
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should be able to use the result directly without needing to do any
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further processing. You may wish to use the
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``django.utils.encoding.iri_to_uri()`` function to help with this if you
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are using unicode strings a lot.
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.. _RFC 2396: http://www.ietf.org/rfc/rfc2396.txt
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The ``permalink`` decorator
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The problem with the way we wrote ``get_absolute_url()`` above is that it
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slightly violates the DRY principle: the URL for this object is defined both
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in the URLConf file and in the model.
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You can further decouple your models from the URLconf using the ``permalink``
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decorator:
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.. function:: django.db.models.permalink()
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This decorator is passed the view function, a list of positional parameters and
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(optionally) a dictionary of named parameters. Django then works out the correct
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full URL path using the URLconf, substituting the parameters you have given into
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the URL. For example, if your URLconf contained a line such as::
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(r'^people/(\d+)/$', 'people.views.details'),
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...your model could have a ``get_absolute_url`` method that looked like this::
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from django.db import models
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@models.permalink
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def get_absolute_url(self):
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return ('people.views.details', [str(self.id)])
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Similarly, if you had a URLconf entry that looked like::
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(r'/archive/(?P<year>\d{4})/(?P<month>\d{1,2})/(?P<day>\d{1,2})/$', archive_view)
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...you could reference this using ``permalink()`` as follows::
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@models.permalink
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def get_absolute_url(self):
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return ('archive_view', (), {
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'year': self.created.year,
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'month': self.created.month,
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'day': self.created.day})
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Notice that we specify an empty sequence for the second parameter in this case,
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because we only want to pass keyword parameters, not positional ones.
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In this way, you're tying the model's absolute URL to the view that is used
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to display it, without repeating the URL information anywhere. You can still
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use the ``get_absolute_url`` method in templates, as before.
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In some cases, such as the use of generic views or the re-use of
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custom views for multiple models, specifying the view function may
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confuse the reverse URL matcher (because multiple patterns point to
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the same view).
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For that problem, Django has **named URL patterns**. Using a named
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URL pattern, it's possible to give a name to a pattern, and then
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reference the name rather than the view function. A named URL
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pattern is defined by replacing the pattern tuple by a call to
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the ``url`` function)::
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from django.conf.urls.defaults import *
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url(r'^people/(\d+)/$',
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'django.views.generic.list_detail.object_detail',
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name='people_view'),
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...and then using that name to perform the reverse URL resolution instead
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of the view name::
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from django.db.models import permalink
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def get_absolute_url(self):
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return ('people_view', [str(self.id)])
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get_absolute_url = permalink(get_absolute_url)
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More details on named URL patterns are in the :ref:`URL dispatch documentation
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<topics-http-urls>`.
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Extra instance methods
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======================
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In addition to ``save()``, ``delete()``, a model object might get any or all
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of the following methods:
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get_FOO_display()
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-----------------
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For every field that has ``choices`` set, the object will have a
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``get_FOO_display()`` method, where ``FOO`` is the name of the field. This
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method returns the "human-readable" value of the field. For example, in the
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following model::
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GENDER_CHOICES = (
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('M', 'Male'),
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('F', 'Female'),
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)
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class Person(models.Model):
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name = models.CharField(max_length=20)
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gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
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...each ``Person`` instance will have a ``get_gender_display()`` method. Example::
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>>> p = Person(name='John', gender='M')
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>>> p.save()
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>>> p.gender
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'M'
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>>> p.get_gender_display()
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'Male'
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get_next_by_FOO(\**kwargs) and get_previous_by_FOO(\**kwargs)
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-------------------------------------------------------------
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For every ``DateField`` and ``DateTimeField`` that does not have ``null=True``,
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the object will have ``get_next_by_FOO()`` and ``get_previous_by_FOO()``
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methods, where ``FOO`` is the name of the field. This returns the next and
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previous object with respect to the date field, raising the appropriate
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``DoesNotExist`` exception when appropriate.
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Both methods accept optional keyword arguments, which should be in the format
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described in :ref:`Field lookups <field-lookups>`.
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Note that in the case of identical date values, these methods will use the ID
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as a fallback check. This guarantees that no records are skipped or duplicated.
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