Managers¶
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 Making queries;
this document specifically touches on model options that customize Manager
behavior.
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¶
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 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
with connection.cursor() as 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, on_delete=models.CASCADE)
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 a manager’s initial QuerySet
¶
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().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().get_queryset().filter(role='A')
class EditorManager(models.Manager):
def get_queryset(self):
return super().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.
Default managers¶
-
Model.
_default_manager
¶
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
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.
You can specify a custom default manager using Meta.default_manager_name
.
If you’re writing some code that must handle an unknown model, for example, in
a third-party app that implements a generic view, use this manager (or
_base_manager
) rather than assuming the model has an objects
manager.
Base managers¶
-
Model.
_base_manager
¶
Don’t filter away any results in this type of manager subclass¶
This manager is used 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 a base 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
.
Creating a manager with QuerySet
methods¶
In lieu of the above approach which requires duplicating methods on both the
QuerySet
and the Manager
, 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 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 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 toFalse
are always copied. - Methods with a
queryset_only
attribute set toTrue
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 advanced 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 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)()
You may also store the generated class into a variable:
CustomManager = BaseManager.from_queryset(CustomQuerySet)
class MyModel(models.Model):
objects = CustomManager()
Custom managers and model inheritance¶
Here’s how Django handles custom managers and model inheritance:
- Managers from 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).
- If no managers are declared on a model and/or its parents, Django
automatically creates the
objects
manager. - The default manager on a class is either the one chosen with
Meta.default_manager_name
, or the first manager declared on the model, or the default manager of the first parent model.
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.