django/tests/modeltests/custom_managers/models.py

"""
23. Giving models a custom manager

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.
"""

from django.db import models

# An example of a custom manager called "objects".

class PersonManager(models.Manager):
    def get_fun_people(self):
        return self.filter(fun=True)

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)
    fun = models.BooleanField()
    objects = PersonManager()

    def __unicode__(self):
        return u"%s %s" % (self.first_name, self.last_name)

# An example of a custom manager that sets get_query_set().

class PublishedBookManager(models.Manager):
    def get_query_set(self):
        return super(PublishedBookManager, self).get_query_set().filter(is_published=True)

class Book(models.Model):
    title = models.CharField(max_length=50)
    author = models.CharField(max_length=30)
    is_published = models.BooleanField()
    published_objects = PublishedBookManager()
    authors = models.ManyToManyField(Person, related_name='books')

    def __unicode__(self):
        return self.title

# An example of providing multiple custom managers.

class FastCarManager(models.Manager):
    def get_query_set(self):
        return super(FastCarManager, self).get_query_set().filter(top_speed__gt=150)

class Car(models.Model):
    name = models.CharField(max_length=10)
    mileage = models.IntegerField()
    top_speed = models.IntegerField(help_text="In miles per hour.")
    cars = models.Manager()
    fast_cars = FastCarManager()

    def __unicode__(self):
        return self.name

__test__ = {'API_TESTS':"""
>>> p1 = Person(first_name='Bugs', last_name='Bunny', fun=True)
>>> p1.save()
>>> p2 = Person(first_name='Droopy', last_name='Dog', fun=False)
>>> p2.save()
>>> Person.objects.get_fun_people()
[<Person: Bugs Bunny>]

# The RelatedManager used on the 'books' descriptor extends the default manager
>>> from modeltests.custom_managers.models import PublishedBookManager
>>> isinstance(p2.books, PublishedBookManager)
True

>>> b1 = Book(title='How to program', author='Rodney Dangerfield', is_published=True)
>>> b1.save()
>>> b2 = Book(title='How to be smart', author='Albert Einstein', is_published=False)
>>> b2.save()

# The default manager, "objects", doesn't exist,
# because a custom one was provided.
>>> Book.objects
Traceback (most recent call last):
    ...
AttributeError: type object 'Book' has no attribute 'objects'

# The RelatedManager used on the 'authors' descriptor extends the default manager
>>> from modeltests.custom_managers.models import PersonManager
>>> isinstance(b2.authors, PersonManager)
True

>>> Book.published_objects.all()
[<Book: How to program>]

>>> c1 = Car(name='Corvette', mileage=21, top_speed=180)
>>> c1.save()
>>> c2 = Car(name='Neon', mileage=31, top_speed=100)
>>> c2.save()
>>> Car.cars.order_by('name')
[<Car: Corvette>, <Car: Neon>]
>>> Car.fast_cars.all()
[<Car: Corvette>]

# Each model class gets a "_default_manager" attribute, which is a reference
# to the first manager defined in the class. In this case, it's "cars".
>>> Car._default_manager.order_by('name')
[<Car: Corvette>, <Car: Neon>]
"""}