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.)
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:
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[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(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 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.
A Manager‘s base 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)
...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_query_set() method. 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 database, but Book.dahl_objects.all() will only return the ones written by Roald Dahl.
Of course, because get_query_set() 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 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.men.all(), Person.women.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 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_query_set() results in an inability to retrieve objects you’d like to work with.
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.
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.
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.
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 django.db.models.Manager class.
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 django.db.models.Manager.
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 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_query_set() method and filter out any rows, Django will return incorrect results. Don’t do that. A manager that filters results in get_query_set() is not appropriate for use as an automatic manager.
Dec 23, 2012