399 lines
16 KiB
Plaintext
399 lines
16 KiB
Plaintext
========
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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 in :doc:`/topics/db/queries`;
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this document specifically touches on model options that customize ``Manager``
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behavior.
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.. _manager-names:
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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 ``Manager``
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Django encounters (in the order in which they're defined in the model) has a
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special status. Django interprets the first ``Manager`` defined in a class as
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the "default" ``Manager``, and several parts of Django
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(including :djadmin:`dumpdata`) will use that ``Manager``
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exclusively for that model. As a result, it's a good idea to be careful in
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your choice of default manager in order to avoid a situation where overriding
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``get_query_set()`` results in an inability to retrieve objects you'd like to
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work with.
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.. _managers-for-related-objects:
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Using managers for related object access
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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By default, Django uses an instance of a "plain" manager class when accessing
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related objects (i.e. ``choice.poll``), not the default manager on the related
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object. This is because Django needs to be able to retrieve the related
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object, even if it would otherwise be filtered out (and hence be inaccessible)
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by the default manager.
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If the normal plain manager class (:class:`django.db.models.Manager`) is not
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appropriate for your circumstances, you can force Django to use the same class
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as the default manager for your model by setting the `use_for_related_fields`
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attribute on the manager class. This is documented fully below_.
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.. _below: manager-types_
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.. _custom-managers-and-inheritance:
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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 behavior 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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These rules provide the necessary flexibility if you want to install a
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collection of custom managers on a group of models, via an abstract base
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class, but still customize the default manager. For example, suppose you have
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this base class::
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class AbstractBase(models.Model):
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...
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objects = CustomManager()
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class Meta:
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abstract = True
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If you use this directly in a subclass, ``objects`` will be the default
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manager if you declare no managers in the base class::
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class ChildA(AbstractBase):
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...
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# This class has CustomManager as the default manager.
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If you want to inherit from ``AbstractBase``, but provide a different default
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manager, you can provide the default manager on the child class::
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class ChildB(AbstractBase):
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...
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# An explicit default manager.
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default_manager = OtherManager()
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Here, ``default_manager`` is the default. The ``objects`` manager is
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still available, since it's inherited. It just isn't used as the default.
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Finally for this example, suppose you want to add extra managers to the child
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class, but still use the default from ``AbstractBase``. You can't add the new
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manager directly in the child class, as that would override the default and you would
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have to also explicitly include all the managers from the abstract base class.
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The solution is to put the extra managers in another base class and introduce
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it into the inheritance hierarchy *after* the defaults::
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class ExtraManager(models.Model):
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extra_manager = OtherManager()
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class Meta:
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abstract = True
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class ChildC(AbstractBase, ExtraManager):
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...
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# Default manager is CustomManager, but OtherManager is
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# also available via the "extra_manager" attribute.
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Implementation concerns
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-----------------------
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Whatever features you add to your custom ``Manager``, it must be
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possible to make a shallow copy of a ``Manager`` instance; i.e., the
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following code must work::
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>>> import copy
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>>> manager = MyManager()
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>>> my_copy = copy.copy(manager)
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Django makes shallow copies of manager objects during certain queries;
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if your Manager cannot be copied, those queries will fail.
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This won't be an issue for most custom managers. If you are just
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adding simple methods to your ``Manager``, it is unlikely that you
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will inadvertently make instances of your ``Manager`` uncopyable.
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However, if you're overriding ``__getattr__`` or some other private
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method of your ``Manager`` object that controls object state, you
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should ensure that you don't affect the ability of your ``Manager`` to
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be copied.
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.. _manager-types:
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Controlling automatic Manager types
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===================================
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This document has already mentioned a couple of places where Django creates a
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manager class for you: `default managers`_ and the "plain" manager used to
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`access related objects`_. There are other places in the implementation of
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Django where temporary plain managers are needed. Those automatically created
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managers will normally be instances of the :class:`django.db.models.Manager`
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class.
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.. _default managers: manager-names_
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.. _access related objects: managers-for-related-objects_
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Throughout this section, we will use the term "automatic manager" to mean a
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manager that Django creates for you -- either as a default manager on a model
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with no managers, or to use temporarily when accessing related objects.
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Sometimes this default class won't be the right choice. One example is in the
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:mod:`django.contrib.gis` application that ships with Django itself. All ``gis``
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models must use a special manager class (:class:`~django.contrib.gis.db.models.GeoManager`)
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because they need a special queryset (:class:`~django.contrib.gis.db.models.GeoQuerySet`)
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to be used for interacting with the database. It turns out that models which require
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a special manager like this need to use the same manager class wherever an automatic
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manager is created.
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Django provides a way for custom manager developers to say that their manager
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class should be used for automatic managers whenever it is the default manager
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on a model. This is done by setting the ``use_for_related_fields`` attribute on
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the manager class::
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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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If this attribute is set on the *default* manager for a model (only the
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default manager is considered in these situations), Django will use that class
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whenever it needs to automatically create a manager for the class. Otherwise,
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it will use :class:`django.db.models.Manager`.
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.. admonition:: Historical Note
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Given the purpose for which it's used, the name of this attribute
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(``use_for_related_fields``) might seem a little odd. Originally, the
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attribute only controlled the type of manager used for related field
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access, which is where the name came from. As it became clear the concept
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was more broadly useful, the name hasn't been changed. This is primarily
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so that existing code will :doc:`continue to work </misc/api-stability>` in
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future Django versions.
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Writing correct Managers for use in automatic Manager instances
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---------------------------------------------------------------
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As already suggested by the `django.contrib.gis` example, above, the
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``use_for_related_fields`` feature is primarily for managers that need to
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return a custom ``QuerySet`` subclass. In providing this functionality in your
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manager, there are a couple of things to remember.
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Do not filter away any results in this type of manager subclass
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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One reason an automatic manager is used is to access objects that are related
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to from some other model. In those situations, Django has to be able to see
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all the objects for the model it is fetching, so that *anything* which is
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referred to can be retrieved.
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If you override the ``get_query_set()`` method and filter out any rows, Django
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will return incorrect results. Don't do that. A manager that filters results
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in ``get_query_set()`` is not appropriate for use as an automatic manager.
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Set ``use_for_related_fields`` when you define the class
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The ``use_for_related_fields`` attribute must be set on the manager *class*, not
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on an *instance* of the class. The earlier example shows the correct way to set
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it, whereas the following will not work::
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# BAD: Incorrect code
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class MyManager(models.Manager):
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...
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# Sets the attribute on an instance of MyManager. Django will
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# ignore this setting.
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mgr = MyManager()
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mgr.use_for_related_fields = True
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class MyModel(models.Model):
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...
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objects = mgr
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# End of incorrect code.
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You also shouldn't change the attribute on the class object after it has been
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used in a model, since the attribute's value is processed when the model class
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is created and not subsequently reread. Set the attribute on the manager class
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when it is first defined, as in the initial example of this section and
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everything will work smoothly.
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