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Also added documentation describing how manager inheritance works (and when manager aren't inherited). Based on some patches from sebastian_noack and emulbreh. git-svn-id: http://code.djangoproject.com/svn/django/trunk@8851 bcc190cf-cafb-0310-a4f2-bffc1f526a37
224 lines
8.8 KiB
Plaintext
224 lines
8.8 KiB
Plaintext
.. _topics-db-managers:
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========
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Managers
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========
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.. currentmodule:: django.db.models
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.. class:: Manager()
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A ``Manager`` is the interface through which database query operations are
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provided to Django models. At least one ``Manager`` exists for every model in
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a Django application.
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The way ``Manager`` classes work is documented :ref:`topics-db-queries`; this
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document specifically touches on model options that customize ``Manager``
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behavior.
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Manager names
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=============
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By default, Django adds a ``Manager`` with the name ``objects`` to every Django
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model class. However, if you want to use ``objects`` as a field name, or if you
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want to use a name other than ``objects`` for the ``Manager``, you can rename
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it on a per-model basis. To rename the ``Manager`` for a given class, define a
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class attribute of type ``models.Manager()`` on that model. For example::
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from django.db import models
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class Person(models.Model):
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#...
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people = models.Manager()
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Using this example model, ``Person.objects`` will generate an
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``AttributeError`` exception, but ``Person.people.all()`` will provide a list
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of all ``Person`` objects.
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.. _custom-managers:
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Custom Managers
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===============
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You can use a custom ``Manager`` in a particular model by extending the base
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``Manager`` class and instantiating your custom ``Manager`` in your model.
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There are two reasons you might want to customize a ``Manager``: to add extra
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``Manager`` methods, and/or to modify the initial ``QuerySet`` the ``Manager``
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returns.
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Adding extra Manager methods
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----------------------------
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Adding extra ``Manager`` methods is the preferred way to add "table-level"
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functionality to your models. (For "row-level" functionality -- i.e., functions
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that act on a single instance of a model object -- use :ref:`Model methods
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<model-methods>`, not custom ``Manager`` methods.)
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A custom ``Manager`` method can return anything you want. It doesn't have to
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return a ``QuerySet``.
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For example, this custom ``Manager`` offers a method ``with_counts()``, which
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returns a list of all ``OpinionPoll`` objects, each with an extra
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``num_responses`` attribute that is the result of an aggregate query::
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class PollManager(models.Manager):
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def with_counts(self):
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from django.db import connection
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cursor = connection.cursor()
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cursor.execute("""
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SELECT p.id, p.question, p.poll_date, COUNT(*)
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FROM polls_opinionpoll p, polls_response r
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WHERE p.id = r.poll_id
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GROUP BY 1, 2, 3
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ORDER BY 3 DESC""")
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result_list = []
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for row in cursor.fetchall():
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p = self.model(id=row[0], question=row[1], poll_date=row[2])
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p.num_responses = row[3]
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result_list.append(p)
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return result_list
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class OpinionPoll(models.Model):
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question = models.CharField(max_length=200)
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poll_date = models.DateField()
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objects = PollManager()
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class Response(models.Model):
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poll = models.ForeignKey(Poll)
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person_name = models.CharField(max_length=50)
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response = models.TextField()
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With this example, you'd use ``OpinionPoll.objects.with_counts()`` to return
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that list of ``OpinionPoll`` objects with ``num_responses`` attributes.
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Another thing to note about this example is that ``Manager`` methods can
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access ``self.model`` to get the model class to which they're attached.
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Modifying initial Manager QuerySets
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-----------------------------------
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A ``Manager``'s base ``QuerySet`` returns all objects in the system. For
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example, using this model::
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class Book(models.Model):
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title = models.CharField(max_length=100)
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author = models.CharField(max_length=50)
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...the statement ``Book.objects.all()`` will return all books in the database.
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You can override a ``Manager``\'s base ``QuerySet`` by overriding the
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``Manager.get_query_set()`` method. ``get_query_set()`` should return a
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``QuerySet`` with the properties you require.
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For example, the following model has *two* ``Manager``\s -- one that returns
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all objects, and one that returns only the books by Roald Dahl::
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# First, define the Manager subclass.
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class DahlBookManager(models.Manager):
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def get_query_set(self):
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return super(DahlBookManager, self).get_query_set().filter(author='Roald Dahl')
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# Then hook it into the Book model explicitly.
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class Book(models.Model):
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title = models.CharField(max_length=100)
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author = models.CharField(max_length=50)
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objects = models.Manager() # The default manager.
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dahl_objects = DahlBookManager() # The Dahl-specific manager.
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With this sample model, ``Book.objects.all()`` will return all books in the
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database, but ``Book.dahl_objects.all()`` will only return the ones written by
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Roald Dahl.
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Of course, because ``get_query_set()`` returns a ``QuerySet`` object, you can
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use ``filter()``, ``exclude()`` and all the other ``QuerySet`` methods on it.
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So these statements are all legal::
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Book.dahl_objects.all()
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Book.dahl_objects.filter(title='Matilda')
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Book.dahl_objects.count()
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This example also pointed out another interesting technique: using multiple
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managers on the same model. You can attach as many ``Manager()`` instances to
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a model as you'd like. This is an easy way to define common "filters" for your
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models.
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For example::
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class MaleManager(models.Manager):
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def get_query_set(self):
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return super(MaleManager, self).get_query_set().filter(sex='M')
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class FemaleManager(models.Manager):
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def get_query_set(self):
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return super(FemaleManager, self).get_query_set().filter(sex='F')
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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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sex = models.CharField(max_length=1, choices=(('M', 'Male'), ('F', 'Female')))
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people = models.Manager()
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men = MaleManager()
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women = FemaleManager()
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This example allows you to request ``Person.men.all()``, ``Person.women.all()``,
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and ``Person.people.all()``, yielding predictable results.
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If you use custom ``Manager`` objects, take note that the first
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``Manager`` Django encounters (in the order in which they're defined
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in the model) has a special status. Django interprets this first
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``Manager`` defined in a class as the "default" ``Manager``, and
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several parts of Django (though not the admin application) will use
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that ``Manager`` exclusively for that model. As a result, it's often a
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good idea to be careful in your choice of default manager, in order to
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avoid a situation where overriding of ``get_query_set()`` results in
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an inability to retrieve objects you'd like to work with.
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Using managers for related object access
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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By default, Django uses a "bare" (i.e. default) manager when accessing related
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objects (i.e. ``choice.poll``). If this default isn't appropriate for your
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default manager, you can force Django to use a custom manager for related object
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attributes by giving it a ``use_for_related_fields`` property::
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class MyManager(models.Manager)::
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use_for_related_fields = True
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...
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...
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Custom managers and model inheritance
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-------------------------------------
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Class inheritance and model managers aren't quite a perfect match for each
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other. Managers are often specific to the classes they are defined on and
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inheriting them in subclasses isn't necessarily a good idea. Also, because the
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first manager declared is the *default manager*, it is important to allow that
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to be controlled. So here's how Django handles custom managers and
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:ref:`model inheritance <model-inheritance>`:
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1. Managers defined on non-abstract base classes are *not* inherited by
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child classes. If you want to reuse a manager from a non-abstract base,
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redeclare it explicitly on the child class. These sorts of managers are
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likely to be fairly specific to the class they are defined on, so
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inheriting them can often lead to unexpected results (particularly as
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far as the default manager goes). Therefore, they aren't passed onto
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child classes.
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2. Managers from abstract base classes are always inherited by the child
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class, using Python's normal name resolution order (names on the child
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class override all others; then come names on the first parent class,
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and so on). Abstract base classes are designed to capture information
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and behaviour that is common to their child classes. Defining common
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managers is an appropriate part of this common information.
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3. The default manager on a class is either the first manager declared on
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the class, if that exists, or the default manager of the first abstract
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base class in the parent hierarchy, if that exists. If no default
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manager is explicitly declared, Django's normal default manager is
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used.
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