managers.txt

========
Managers
========

.. currentmodule:: django.db.models

.. class:: Manager()

A ``Manager`` is the interface through which database query operations are
provided to Django models. At least one ``Manager`` exists for every model in
a Django application.

The way ``Manager`` classes work is documented in :doc:`/topics/db/queries`;
this document specifically touches on model options that customize ``Manager``
behavior.

.. _manager-names:

Manager names
=============

By default, Django adds a ``Manager`` with the name ``objects`` to every Django
model class. However, if you want to use ``objects`` as a field name, or if you
want to use a name other than ``objects`` for the ``Manager``, you can rename
it on a per-model basis. To rename the ``Manager`` for a given class, define a
class attribute of type ``models.Manager()`` on that model. For example::

    from django.db import models

    class Person(models.Model):
        #...
        people = models.Manager()

Using this example model, ``Person.objects`` will generate an
``AttributeError`` exception, but ``Person.people.all()`` will provide a list
of all ``Person`` objects.

.. _custom-managers:

Custom Managers
===============

You can use a custom ``Manager`` in a particular model by extending the base
``Manager`` class and instantiating your custom ``Manager`` in your model.

There are two reasons you might want to customize a ``Manager``: to add extra
``Manager`` methods, and/or to modify the initial ``QuerySet`` the ``Manager``
returns.

Adding extra Manager methods
----------------------------

Adding extra ``Manager`` methods is the preferred way to add "table-level"
functionality to your models. (For "row-level" functionality -- i.e., functions
that act on a single instance of a model object -- use :ref:`Model methods
<model-methods>`, not custom ``Manager`` methods.)

A custom ``Manager`` method can return anything you want. It doesn't have to
return a ``QuerySet``.

For example, this custom ``Manager`` offers a method ``with_counts()``, which
returns a list of all ``OpinionPoll`` objects, each with an extra
``num_responses`` attribute that is the result of an aggregate query::

    class PollManager(models.Manager):
        def with_counts(self):
            from django.db import connection
            cursor = connection.cursor()
            cursor.execute("""
                SELECT p.id, p.question, p.poll_date, COUNT(*)
                FROM polls_opinionpoll p, polls_response r
                WHERE p.id = r.poll_id
                GROUP BY 1, 2, 3
                ORDER BY 3 DESC""")
            result_list = []
            for row in cursor.fetchall():
                p = self.model(id=row[0], question=row[1], poll_date=row[2])
                p.num_responses = row[3]
                result_list.append(p)
            return result_list

    class OpinionPoll(models.Model):
        question = models.CharField(max_length=200)
        poll_date = models.DateField()
        objects = PollManager()

    class Response(models.Model):
        poll = models.ForeignKey(Poll)
        person_name = models.CharField(max_length=50)
        response = models.TextField()

With this example, you'd use ``OpinionPoll.objects.with_counts()`` to return
that list of ``OpinionPoll`` objects with ``num_responses`` attributes.

Another thing to note about this example is that ``Manager`` methods can
access ``self.model`` to get the model class to which they're attached.

Modifying initial Manager QuerySets
-----------------------------------

A ``Manager``'s base ``QuerySet`` returns all objects in the system. For
example, using this model::

    class Book(models.Model):
        title = models.CharField(max_length=100)
        author = models.CharField(max_length=50)

...the statement ``Book.objects.all()`` will return all books in the database.

You can override a ``Manager``\'s base ``QuerySet`` by overriding the
``Manager.get_query_set()`` method. ``get_query_set()`` should return a
``QuerySet`` with the properties you require.

For example, the following model has *two* ``Manager``\s -- one that returns
all objects, and one that returns only the books by Roald Dahl::

    # First, define the Manager subclass.
    class DahlBookManager(models.Manager):
        def get_query_set(self):
            return super(DahlBookManager, self).get_query_set().filter(author='Roald Dahl')

    # Then hook it into the Book model explicitly.
    class Book(models.Model):
        title = models.CharField(max_length=100)
        author = models.CharField(max_length=50)

        objects = models.Manager() # The default manager.
        dahl_objects = DahlBookManager() # The Dahl-specific manager.

With this sample model, ``Book.objects.all()`` will return all books in the
database, but ``Book.dahl_objects.all()`` will only return the ones written by
Roald Dahl.

Of course, because ``get_query_set()`` returns a ``QuerySet`` object, you can
use ``filter()``, ``exclude()`` and all the other ``QuerySet`` methods on it.
So these statements are all legal::

    Book.dahl_objects.all()
    Book.dahl_objects.filter(title='Matilda')
    Book.dahl_objects.count()

This example also pointed out another interesting technique: using multiple
managers on the same model. You can attach as many ``Manager()`` instances to
a model as you'd like. This is an easy way to define common "filters" for your
models.

For example::

    class MaleManager(models.Manager):
        def get_query_set(self):
            return super(MaleManager, self).get_query_set().filter(sex='M')

    class FemaleManager(models.Manager):
        def get_query_set(self):
            return super(FemaleManager, self).get_query_set().filter(sex='F')

    class Person(models.Model):
        first_name = models.CharField(max_length=50)
        last_name = models.CharField(max_length=50)
        sex = models.CharField(max_length=1, choices=(('M', 'Male'), ('F', 'Female')))
        people = models.Manager()
        men = MaleManager()
        women = FemaleManager()

This example allows you to request ``Person.men.all()``, ``Person.women.all()``,
and ``Person.people.all()``, yielding predictable results.

If you use custom ``Manager`` objects, take note that the first ``Manager``
Django encounters (in the order in which they're defined in the model) has a
special status. Django interprets the first ``Manager`` defined in a class as
the "default" ``Manager``, and several parts of Django
(including :djadmin:`dumpdata`) will use that ``Manager``
exclusively for that model. As a result, it's a good idea to be careful in
your choice of default manager in order to avoid a situation where overriding
``get_query_set()`` results in an inability to retrieve objects you'd like to
work with.

.. _managers-for-related-objects:

Using managers for related object access
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

By default, Django uses an instance of a "plain" manager class when accessing
related objects (i.e. ``choice.poll``), not the default manager on the related
object. This is because Django needs to be able to retrieve the related
object, even if it would otherwise be filtered out (and hence be inaccessible)
by the default manager.

If the normal plain manager class (:class:`django.db.models.Manager`) is not
appropriate for your circumstances, you can force Django to use the same class
as the default manager for your model by setting the `use_for_related_fields`
attribute on the manager class. This is documented fully below_.

.. _below: manager-types_

.. _custom-managers-and-inheritance:

Custom managers and model inheritance
-------------------------------------

Class inheritance and model managers aren't quite a perfect match for each
other. Managers are often specific to the classes they are defined on and
inheriting them in subclasses isn't necessarily a good idea. Also, because the
first manager declared is the *default manager*, it is important to allow that
to be controlled. So here's how Django handles custom managers and
:ref:`model inheritance <model-inheritance>`:

    1. Managers defined on non-abstract base classes are *not* inherited by
       child classes. If you want to reuse a manager from a non-abstract base,
       redeclare it explicitly on the child class. These sorts of managers are
       likely to be fairly specific to the class they are defined on, so
       inheriting them can often lead to unexpected results (particularly as
       far as the default manager goes). Therefore, they aren't passed onto
       child classes.

    2. Managers from abstract base classes are always inherited by the child
       class, using Python's normal name resolution order (names on the child
       class override all others; then come names on the first parent class,
       and so on). Abstract base classes are designed to capture information
       and behavior that is common to their child classes. Defining common
       managers is an appropriate part of this common information.

    3. The default manager on a class is either the first manager declared on
       the class, if that exists, or the default manager of the first abstract
       base class in the parent hierarchy, if that exists. If no default
       manager is explicitly declared, Django's normal default manager is
       used.

These rules provide the necessary flexibility if you want to install a
collection of custom managers on a group of models, via an abstract base
class, but still customize the default manager. For example, suppose you have
this base class::

    class AbstractBase(models.Model):
        ...
        objects = CustomManager()

        class Meta:
            abstract = True

If you use this directly in a subclass, ``objects`` will be the default
manager if you declare no managers in the base class::

    class ChildA(AbstractBase):
        ...
        # This class has CustomManager as the default manager.

If you want to inherit from ``AbstractBase``, but provide a different default
manager, you can provide the default manager on the child class::

    class ChildB(AbstractBase):
        ...
        # An explicit default manager.
        default_manager = OtherManager()

Here, ``default_manager`` is the default. The ``objects`` manager is
still available, since it's inherited. It just isn't used as the default.

Finally for this example, suppose you want to add extra managers to the child
class, but still use the default from ``AbstractBase``. You can't add the new
manager directly in the child class, as that would override the default and you would
have to also explicitly include all the managers from the abstract base class.
The solution is to put the extra managers in another base class and introduce
it into the inheritance hierarchy *after* the defaults::

    class ExtraManager(models.Model):
        extra_manager = OtherManager()

        class Meta:
            abstract = True

    class ChildC(AbstractBase, ExtraManager):
        ...
        # Default manager is CustomManager, but OtherManager is
        # also available via the "extra_manager" attribute.

Implementation concerns
-----------------------

Whatever features you add to your custom ``Manager``, it must be
possible to make a shallow copy of a ``Manager`` instance; i.e., the
following code must work::

    >>> import copy
    >>> manager = MyManager()
    >>> my_copy = copy.copy(manager)

Django makes shallow copies of manager objects during certain queries;
if your Manager cannot be copied, those queries will fail.

This won't be an issue for most custom managers. If you are just
adding simple methods to your ``Manager``, it is unlikely that you
will inadvertently make instances of your ``Manager`` uncopyable.
However, if you're overriding ``__getattr__`` or some other private
method of your ``Manager`` object that controls object state, you
should ensure that you don't affect the ability of your ``Manager`` to
be copied.

.. _manager-types:

Controlling automatic Manager types
===================================

This document has already mentioned a couple of places where Django creates a
manager class for you: `default managers`_ and the "plain" manager used to
`access related objects`_. There are other places in the implementation of
Django where temporary plain managers are needed. Those automatically created
managers will normally be instances of the :class:`django.db.models.Manager`
class.

.. _default managers: manager-names_
.. _access related objects: managers-for-related-objects_

Throughout this section, we will use the term "automatic manager" to mean a
manager that Django creates for you -- either as a default manager on a model
with no managers, or to use temporarily when accessing related objects.

Sometimes this default class won't be the right choice. One example is in the
:mod:`django.contrib.gis` application that ships with Django itself. All ``gis``
models must use a special manager class (:class:`~django.contrib.gis.db.models.GeoManager`)
because they need a special queryset (:class:`~django.contrib.gis.db.models.GeoQuerySet`)
to be used for interacting with the database.  It turns out that models which require
a special manager like this need to use the same manager class wherever an automatic
manager is created.

Django provides a way for custom manager developers to say that their manager
class should be used for automatic managers whenever it is the default manager
on a model. This is done by setting the ``use_for_related_fields`` attribute on
the manager class::

    class MyManager(models.Manager):
        use_for_related_fields = True

        ...

If this attribute is set on the *default* manager for a model (only the
default manager is considered in these situations), Django will use that class
whenever it needs to automatically create a manager for the class.  Otherwise,
it will use :class:`django.db.models.Manager`.

.. admonition:: Historical Note

    Given the purpose for which it's used, the name of this attribute
    (``use_for_related_fields``) might seem a little odd. Originally, the
    attribute only controlled the type of manager used for related field
    access, which is where the name came from. As it became clear the concept
    was more broadly useful, the name hasn't been changed. This is primarily
    so that existing code will :doc:`continue to work </misc/api-stability>` in
    future Django versions.

Writing correct Managers for use in automatic Manager instances
---------------------------------------------------------------

As already suggested by the `django.contrib.gis` example, above, the
``use_for_related_fields`` feature is primarily for managers that need to
return a custom ``QuerySet`` subclass. In providing this functionality in your
manager, there are a couple of things to remember.

Do not filter away any results in this type of manager subclass
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

One reason an automatic manager is used is to access objects that are related
to from some other model. In those situations, Django has to be able to see
all the objects for the model it is fetching, so that *anything* which is
referred to can be retrieved.

If you override the ``get_query_set()`` method and filter out any rows, Django
will return incorrect results. Don't do that. A manager that filters results
in ``get_query_set()`` is not appropriate for use as an automatic manager.

Set ``use_for_related_fields`` when you define the class
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``use_for_related_fields`` attribute must be set on the manager *class*,
object not on an *instance* of the class. The earlier example shows the
correct way to set it, whereas the following will not work::

    # BAD: Incorrect code
    class MyManager(models.Manager):
        ...

    # Sets the attribute on an instance of MyManager. Django will
    # ignore this setting.
    mgr = MyManager()
    mgr.use_for_related_fields = True

    class MyModel(models.Model):
        ...
        objects = mgr

    # End of incorrect code.

You also shouldn't change the attribute on the class object after it has been
used in a model, since the attribute's value is processed when the model class
is created and not subsequently reread. Set the attribute on the manager class
when it is first defined, as in the initial example of this section and
everything will work smoothly.