identity_map.rst

Identity map

This library currently provides a replacement implementation of prefetch_related_objects which uses an identity map to automatically reduce the number of queries performed when prefetching.

For example, considered the following data model:

class Toy(models.Model):
    dog = models.ForeignKey('dogs.Dog')

class Dog(models.Model):
    name = models.CharField()
    favorite_toy = models.ForeignKey('toys.Toy', null=True)

With this library, we get don't need to do a database query to perform the prefetch for favorite_toy since that object had already been fetched as part of the prefetching for toy_set:

>>> dog = Dog.objects.prefetch_related('toys', 'favorite_toy')[0]
SELECT * from dogs_dog limit 1;
SELECT * FROM toys_toy where toys_toy.dog_id IN (1);
>>> dog.favorite_toy is dog.toy_set.all()[0]  # no queries done
True

.. toctree::
   :maxdepth: 2
   :hidden:

   identity_map_usage
   identity_map_comparison