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.
Manager classes work is documented in Making queries; this document specifically touches on model options that customize
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
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
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 method can return anything you want. It doesn’t have to return a
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, question=row, poll_date=row) p.num_responses = row 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
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.
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)
Book.objects.all() will return all books in the database.
You can override a
QuerySet by overriding the
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(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
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.
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.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
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.
If the normal plain manager 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.
QuerySetmethods from the
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
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
editors() directly from the manager
In lieu of the above approach which requires duplicating methods on both the
QuerySet and the
QuerySet.as_manager() can be used to create an instance of
Manager with a copy of a custom
class Person(models.Model): ... people = PersonQuerySet.as_manager()
Manager instance created by
QuerySet.as_manager() will be virtually identical to the
PersonManager from the previous example.
QuerySet method makes sense at the
Manager level; for instance we intentionally prevent the
QuerySet.delete() method from being copied onto the
Methods are copied according to the following rules:
queryset_onlyattribute set to
Falseare always copied.
queryset_onlyattribute set to
Trueare never copied.
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
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
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()
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 model inheritance:
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()
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:
is legal, but:
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
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.
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
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
django.contrib.gis application that ships with Django itself. All
gis models must use a special manager class (
GeoManager) because they need a special queryset (
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
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 continue to work in future Django versions.
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.
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.
use_for_related_fieldswhen you define the class
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.
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