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 Making queries;
this document specifically touches on model options that customize Manager
behavior.
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.
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 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.)
For example, this custom Manager adds a method with_counts():
from django.db import models
from django.db.models.functions import Coalesce
class PollManager(models.Manager):
def with_counts(self):
return self.annotate(
num_responses=Coalesce(models.Count("response"), 0)
)
class OpinionPoll(models.Model):
question = models.CharField(max_length=200)
objects = PollManager()
class Response(models.Model):
poll = models.ForeignKey(OpinionPoll, on_delete=models.CASCADE)
# ...
With this example, you’d use OpinionPoll.objects.with_counts() to get a
QuerySet of OpinionPoll objects with the extra num_responses
attribute attached.
A custom Manager method can return anything you want. It doesn’t have to
return a QuerySet.
Another thing to note is that Manager methods can access self.model to
get the model class to which they’re attached.
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 Managers – 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.
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 a non-repetitive 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.
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.
Model._base_manager¶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.
Therefore, you should not override get_queryset() to filter out any rows.
If you do so, Django will return incomplete results.
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.
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:
queryset_only attribute set to False are always copied.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()¶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 CustomManager(models.Manager):
def manager_only_method(self):
return
class CustomQuerySet(models.QuerySet):
def manager_and_queryset_method(self):
return
class MyModel(models.Model):
objects = CustomManager.from_queryset(CustomQuerySet)()
You may also store the generated class into a variable:
MyManager = CustomManager.from_queryset(CustomQuerySet)
class MyModel(models.Model):
objects = MyManager()
Here’s how Django handles custom managers and model inheritance:
objects manager.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, but 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.
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.
Jul 27, 2022