Fixed #20939 -- Simplified query generation by converting QuerySet to Query.

Thanks Anssi Kääriäinen for the initial patch and Anssi, Simon Charette,
and Josh Smeaton for review.

django/db/models/fields/related_lookups.py

@@ -81,27 +81,18 @@ class RelatedIn(In):
                     AND)
             return root_constraint.as_sql(compiler, connection)
         else:
+            if getattr(self.rhs, '_forced_pk', False):
+                self.rhs.clear_select_clause()
+                if getattr(self.lhs.output_field, 'primary_key', False):
+                    # A case like Restaurant.objects.filter(place__in=restaurant_qs),
+                    # where place is a OneToOneField and the primary key of
+                    # Restaurant.
+                    target_field = self.lhs.field.name
+                else:
+                    target_field = self.lhs.field.target_field.name
+                self.rhs.add_fields([target_field], True)
             return super(RelatedIn, self).as_sql(compiler, connection)
 
-    def __getstate__(self):
-        """
-        Prevent pickling a query with an __in=inner_qs lookup from evaluating
-        inner_qs.
-        """
-        from django.db.models.query import QuerySet  # Avoid circular import
-        state = self.__dict__.copy()
-        if isinstance(self.rhs, QuerySet):
-            state['rhs'] = (self.rhs.__class__, self.rhs.query)
-        return state
-
-    def __setstate__(self, state):
-        self.__dict__.update(state)
-        if isinstance(self.rhs, tuple):
-            queryset_class, query = self.rhs
-            queryset = queryset_class()
-            queryset.query = query
-            self.rhs = queryset
-
 
 class RelatedLookupMixin(object):
     def get_prep_lookup(self):