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
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
Adding extra Manager methods¶
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
For example, this custom
Manager offers a method
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, 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(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
Another thing to note about this example is that
Manager methods can
self.model to get the model class to which they’re attached.
Modifying initial Manager QuerySets¶
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)
Book.objects.all() will return all books in the database.
You can override a
QuerySet by overriding the
get_query_set() 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_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
Book.dahl_objects.all() will only return the ones written by
Of course, because
get_query_set() returns a
QuerySet object, you can
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
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.people.all(), yielding predictable results.
If you use custom
Manager objects, take note that the first
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
Manager, and several parts of Django
dumpdata) will use that
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
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 model inheritance:
- 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.
- 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.
- 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()
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.
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
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
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
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
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
models must use a special manager class (
because they need a special queryset (
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.
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
get_query_set() is not appropriate for use as an automatic manager.