Fixed #19816 -- Pre-evaluate querysets used in direct relation assignments.

Since assignments on M2M or reverse FK descriptors is composed of a `clear()`,
followed by an `add()`, `clear()` could potentially affect the value of the
assigned queryset before the `add()` step; pre-evaluating it solves the problem.

This patch fixes the issue for ForeignRelatedObjectsDescriptor,
ManyRelatedObjectsDescriptor, and ReverseGenericRelatedObjectsDescriptor.
It completes 6cb6e1 which addressed ReverseManyRelatedObjectsDescriptor.

2 files changed, 15 insertions, 7 deletions

diff --git a/django/contrib/contenttypes/fields.py b/django/contrib/contenttypes/fields.py
index 7e91957546..6ea225191a 100644
--- a/django/contrib/contenttypes/fields.py
+++ b/django/contrib/contenttypes/fields.py
@@ -393,8 +393,11 @@ class ReverseGenericRelatedObjectsDescriptor(object):
         return manager
 
     def __set__(self, instance, value):
-        manager = self.__get__(instance)
+        # Force evaluation of `value` in case it's a queryset whose
+        # value could be affected by `manager.clear()`. Refs #19816.
+        value = tuple(value)
 
+        manager = self.__get__(instance)
         db = router.db_for_write(manager.model, instance=manager.instance)
         with transaction.atomic(using=db, savepoint=False):
             manager.clear()
diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index 523b64a7e2..a789772dce 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -763,8 +763,11 @@ class ForeignRelatedObjectsDescriptor(object):
         return self.related_manager_cls(instance)
 
     def __set__(self, instance, value):
-        manager = self.__get__(instance)
+        # Force evaluation of `value` in case it's a queryset whose
+        # value could be affected by `manager.clear()`. Refs #19816.
+        value = tuple(value)
 
+        manager = self.__get__(instance)
         db = router.db_for_write(manager.model, instance=manager.instance)
         with transaction.atomic(using=db, savepoint=False):
             # If the foreign key can support nulls, then completely clear the related set.
@@ -1113,8 +1116,11 @@ class ManyRelatedObjectsDescriptor(object):
             opts = self.related.field.rel.through._meta
             raise AttributeError("Cannot set values on a ManyToManyField which specifies an intermediary model. Use %s.%s's Manager instead." % (opts.app_label, opts.object_name))
 
-        manager = self.__get__(instance)
+        # Force evaluation of `value` in case it's a queryset whose
+        # value could be affected by `manager.clear()`. Refs #19816.
+        value = tuple(value)
 
+        manager = self.__get__(instance)
         db = router.db_for_write(manager.through, instance=manager.instance)
         with transaction.atomic(using=db, savepoint=False):
             manager.clear()
@@ -1170,12 +1176,11 @@ class ReverseManyRelatedObjectsDescriptor(object):
             opts = self.field.rel.through._meta
             raise AttributeError("Cannot set values on a ManyToManyField which specifies an intermediary model.  Use %s.%s's Manager instead." % (opts.app_label, opts.object_name))
 
-        manager = self.__get__(instance)
-
-        # clear() can change expected output of 'value' queryset, we force evaluation
-        # of queryset before clear; ticket #19816
+        # Force evaluation of `value` in case it's a queryset whose
+        # value could be affected by `manager.clear()`. Refs #19816.
         value = tuple(value)
 
+        manager = self.__get__(instance)
         db = router.db_for_write(manager.through, instance=manager.instance)
         with transaction.atomic(using=db, savepoint=False):
             manager.clear()