Fixed #17001 -- Custom querysets for prefetch_related.

This patch introduces the Prefetch object which allows customizing prefetch
operations.

This enables things like filtering prefetched relations, calling select_related
from a prefetched relation, or prefetching the same relation multiple times
with different querysets.

When a Prefetch instance specifies a to_attr argument, the result is stored
in a list rather than a QuerySet. This has the fortunate consequence of being
significantly faster. The preformance improvement is due to the fact that we
save the costly creation of a QuerySet instance.

Thanks @akaariai for the original patch and @bmispelon and @timgraham
for the reviews.

1 files changed, 1 insertions, 1 deletions

diff --git a/django/db/models/__init__.py b/django/db/models/__init__.py
index 13785ed17a..e2f7348ef0 100644
--- a/django/db/models/__init__.py
+++ b/django/db/models/__init__.py
@@ -4,7 +4,7 @@ from django.core.exceptions import ObjectDoesNotExist, ImproperlyConfigured  # N
 from django.db.models.loading import (  # NOQA
     get_apps, get_app_path, get_app_paths, get_app, get_models, get_model,
     register_models, UnavailableApp)
-from django.db.models.query import Q, QuerySet  # NOQA
+from django.db.models.query import Q, QuerySet, Prefetch  # NOQA
 from django.db.models.expressions import F  # NOQA
 from django.db.models.manager import Manager  # NOQA
 from django.db.models.base import Model  # NOQA