mirror of
https://github.com/django/django.git
synced 2024-11-29 22:56:46 +01:00
538 lines
21 KiB
Plaintext
538 lines
21 KiB
Plaintext
========
|
||
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::
|
||
|
||
from django.db import models
|
||
|
||
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 p.id, p.question, p.poll_date
|
||
ORDER BY p.poll_date 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(OpinionPoll)
|
||
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::
|
||
|
||
from django.db import models
|
||
|
||
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_queryset()`` method. ``get_queryset()`` 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_queryset(self):
|
||
return super(DahlBookManager, self).get_queryset().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_queryset()`` 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 AuthorManager(models.Manager):
|
||
def get_queryset(self):
|
||
return super(AuthorManager, self).get_queryset().filter(role='A')
|
||
|
||
class EditorManager(models.Manager):
|
||
def get_queryset(self):
|
||
return super(EditorManager, self).get_queryset().filter(role='E')
|
||
|
||
class Person(models.Model):
|
||
first_name = models.CharField(max_length=50)
|
||
last_name = models.CharField(max_length=50)
|
||
role = models.CharField(max_length=1, choices=(('A', _('Author')), ('E', _('Editor'))))
|
||
people = models.Manager()
|
||
authors = AuthorManager()
|
||
editors = EditorManager()
|
||
|
||
This example allows you to request ``Person.authors.all()``, ``Person.editors.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_queryset()`` 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_
|
||
|
||
.. _calling-custom-queryset-methods-from-manager:
|
||
|
||
Calling custom ``QuerySet`` methods from the ``Manager``
|
||
--------------------------------------------------------
|
||
|
||
While most methods from the standard ``QuerySet`` are accessible directly from
|
||
the ``Manager``, this is only the case for the extra methods defined on a
|
||
custom ``QuerySet`` if you also implement them on the ``Manager``::
|
||
|
||
class PersonQuerySet(models.QuerySet):
|
||
def authors(self):
|
||
return self.filter(role='A')
|
||
|
||
def editors(self):
|
||
return self.filter(role='E')
|
||
|
||
class PersonManager(models.Manager):
|
||
def get_queryset(self):
|
||
return PersonQuerySet(self.model, using=self._db)
|
||
|
||
def authors(self):
|
||
return self.get_queryset().authors()
|
||
|
||
def editors(self):
|
||
return self.get_queryset().editors()
|
||
|
||
class Person(models.Model):
|
||
first_name = models.CharField(max_length=50)
|
||
last_name = models.CharField(max_length=50)
|
||
role = models.CharField(max_length=1, choices=(('A', _('Author')), ('E', _('Editor'))))
|
||
people = PersonManager()
|
||
|
||
This example allows you to call both ``authors()`` and ``editors()`` directly from
|
||
the manager ``Person.people``.
|
||
|
||
.. _create-manager-with-queryset-methods:
|
||
|
||
Creating ``Manager`` with ``QuerySet`` methods
|
||
----------------------------------------------
|
||
|
||
.. versionadded:: 1.7
|
||
|
||
In lieu of the above approach which requires duplicating methods on both the
|
||
``QuerySet`` and the ``Manager``, :meth:`QuerySet.as_manager()
|
||
<django.db.models.query.QuerySet.as_manager>` can be used to create an instance
|
||
of ``Manager`` with a copy of a custom ``QuerySet``’s methods::
|
||
|
||
class Person(models.Model):
|
||
...
|
||
people = PersonQuerySet.as_manager()
|
||
|
||
The ``Manager`` instance created by :meth:`QuerySet.as_manager()
|
||
<django.db.models.query.QuerySet.as_manager>` will be virtually
|
||
identical to the ``PersonManager`` from the previous example.
|
||
|
||
Not every ``QuerySet`` method makes sense at the ``Manager`` level; for
|
||
instance we intentionally prevent the :meth:`QuerySet.delete()
|
||
<django.db.models.query.QuerySet.delete>` method from being copied onto
|
||
the ``Manager`` class.
|
||
|
||
Methods are copied according to the following rules:
|
||
|
||
- Public methods are copied by default.
|
||
- Private methods (starting with an underscore) are not copied by default.
|
||
- Methods with a ``queryset_only`` attribute set to ``False`` are always copied.
|
||
- Methods with a ``queryset_only`` attribute set to ``True`` are never copied.
|
||
|
||
For example::
|
||
|
||
class CustomQuerySet(models.QuerySet):
|
||
# Available on both Manager and QuerySet.
|
||
def public_method(self):
|
||
return
|
||
|
||
# Available only on QuerySet.
|
||
def _private_method(self):
|
||
return
|
||
|
||
# Available only on QuerySet.
|
||
def opted_out_public_method(self):
|
||
return
|
||
opted_out_public_method.queryset_only = True
|
||
|
||
# Available on both Manager and QuerySet.
|
||
def _opted_in_private_method(self):
|
||
return
|
||
_opted_in_private_method.queryset_only = False
|
||
|
||
from_queryset
|
||
~~~~~~~~~~~~~
|
||
|
||
.. classmethod:: from_queryset(queryset_class)
|
||
|
||
For advance usage you might want both a custom ``Manager`` and a custom
|
||
``QuerySet``. You can do that by calling ``Manager.from_queryset()`` which
|
||
returns a *subclass* of your base ``Manager`` with a copy of the custom
|
||
``QuerySet`` methods::
|
||
|
||
class BaseManager(models.Manager):
|
||
def __init__(self, *args, **kwargs):
|
||
...
|
||
|
||
def manager_only_method(self):
|
||
return
|
||
|
||
class CustomQuerySet(models.QuerySet):
|
||
def manager_and_queryset_method(self):
|
||
return
|
||
|
||
class MyModel(models.Model):
|
||
objects = BaseManager.from_queryset(CustomQueryset)(*args, **kwargs)
|
||
|
||
You may also store the generated class into a variable::
|
||
|
||
CustomManager = BaseManager.from_queryset(CustomQueryset)
|
||
|
||
class MyModel(models.Model):
|
||
objects = CustomManager(*args, **kwargs)
|
||
|
||
.. _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.
|
||
pass
|
||
|
||
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.
|
||
pass
|
||
|
||
Note that while you can *define* a custom manager on the abstract model, you
|
||
can't *invoke* any methods using the abstract model. That is::
|
||
|
||
ClassA.objects.do_something()
|
||
|
||
is legal, but::
|
||
|
||
AbstractBase.objects.do_something()
|
||
|
||
will raise an exception. This is because managers are intended to encapsulate
|
||
logic for managing collections of objects. Since you can't have a collection of
|
||
abstract objects, it doesn't make sense to be managing them. If you have
|
||
functionality that applies to the abstract model, you should put that functionality
|
||
in a ``staticmethod`` or ``classmethod`` on the abstract model.
|
||
|
||
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 :mod:`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_queryset()`` method and filter out any rows, Django
|
||
will return incorrect results. Don't do that. A manager that filters results
|
||
in ``get_queryset()`` 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*, 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):
|
||
# ...
|
||
pass
|
||
|
||
# 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.
|