Fixed #20577 -- Deferred filtering of prefetched related querysets.

Added internal interface to QuerySet that allows to defer next filter
call till .query is accessed. Used it to optimize prefetch_related().

Thanks Simon Charette for the review.

2 files changed, 26 insertions, 4 deletions

diff --git a/django/db/models/fields/related_descriptors.py b/django/db/models/fields/related_descriptors.py
index 0b51b68ebe..8468a0b01f 100644
--- a/django/db/models/fields/related_descriptors.py
+++ b/django/db/models/fields/related_descriptors.py
@@ -883,6 +883,7 @@ def create_forward_many_to_many_manager(superclass, rel, reverse):
             queryset._add_hints(instance=self.instance)
             if self._db:
                 queryset = queryset.using(self._db)
+            queryset._defer_next_filter = True
             return queryset._next_is_sticky().filter(**self.core_filters)
 
         def _remove_prefetched_objects(self):
diff --git a/django/db/models/query.py b/django/db/models/query.py
index b855720f56..b61c76f80f 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -189,7 +189,7 @@ class QuerySet:
         self.model = model
         self._db = using
         self._hints = hints or {}
-        self.query = query or sql.Query(self.model)
+        self._query = query or sql.Query(self.model)
         self._result_cache = None
         self._sticky_filter = False
         self._for_write = False
@@ -198,6 +198,20 @@ class QuerySet:
         self._known_related_objects = {}  # {rel_field: {pk: rel_obj}}
         self._iterable_class = ModelIterable
         self._fields = None
+        self._defer_next_filter = False
+        self._deferred_filter = None
+
+    @property
+    def query(self):
+        if self._deferred_filter:
+            negate, args, kwargs = self._deferred_filter
+            self._filter_or_exclude_inplace(negate, *args, **kwargs)
+            self._deferred_filter = None
+        return self._query
+
+    @query.setter
+    def query(self, value):
+        self._query = value
 
     def as_manager(cls):
         # Address the circular dependency between `Queryset` and `Manager`.
@@ -914,12 +928,19 @@ class QuerySet:
                 "Cannot filter a query once a slice has been taken."
 
         clone = self._chain()
-        if negate:
-            clone.query.add_q(~Q(*args, **kwargs))
+        if self._defer_next_filter:
+            self._defer_next_filter = False
+            clone._deferred_filter = negate, args, kwargs
         else:
-            clone.query.add_q(Q(*args, **kwargs))
+            clone._filter_or_exclude_inplace(negate, *args, **kwargs)
         return clone
 
+    def _filter_or_exclude_inplace(self, negate, *args, **kwargs):
+        if negate:
+            self._query.add_q(~Q(*args, **kwargs))
+        else:
+            self._query.add_q(Q(*args, **kwargs))
+
     def complex_filter(self, filter_obj):
         """
         Return a new QuerySet instance with filter_obj added to the filters.