diff --git a/django/db/models/fields/related_descriptors.py b/django/db/models/fields/related_descriptors.py
index 02b5bcb62e..96b8a37583 100644
--- a/django/db/models/fields/related_descriptors.py
+++ b/django/db/models/fields/related_descriptors.py
@@ -1051,6 +1051,44 @@ def create_forward_many_to_many_manager(superclass, rel, reverse):
             return obj, created
         update_or_create.alters_data = True
 
+        def _get_missing_target_ids(self, source_field_name, target_field_name, db, objs):
+            """
+            Return the subset of ids of `objs` that aren't already assigned to
+            this relationship.
+            """
+            from django.db.models import Model
+            target_ids = set()
+            target_field = self.through._meta.get_field(target_field_name)
+            for obj in objs:
+                if isinstance(obj, self.model):
+                    if not router.allow_relation(obj, self.instance):
+                        raise ValueError(
+                            'Cannot add "%r": instance is on database "%s", '
+                            'value is on database "%s"' %
+                            (obj, self.instance._state.db, obj._state.db)
+                        )
+                    target_id = target_field.get_foreign_related_value(obj)[0]
+                    if target_id is None:
+                        raise ValueError(
+                            'Cannot add "%r": the value for field "%s" is None' %
+                            (obj, target_field_name)
+                        )
+                    target_ids.add(target_id)
+                elif isinstance(obj, Model):
+                    raise TypeError(
+                        "'%s' instance expected, got %r" %
+                        (self.model._meta.object_name, obj)
+                    )
+                else:
+                    target_ids.add(obj)
+            vals = self.through._default_manager.using(db).values_list(
+                target_field_name, flat=True
+            ).filter(**{
+                source_field_name: self.related_val[0],
+                '%s__in' % target_field_name: target_ids,
+            })
+            return target_ids.difference(vals)
+
         def _add_items(self, source_field_name, target_field_name, *objs, through_defaults=None):
             # source_field_name: the PK fieldname in join table for the source object
             # target_field_name: the PK fieldname in join table for the target object
@@ -1058,40 +1096,9 @@ def create_forward_many_to_many_manager(superclass, rel, reverse):
             through_defaults = through_defaults or {}
 
             # If there aren't any objects, there is nothing to do.
-            from django.db.models import Model
             if objs:
-                new_ids = set()
-                for obj in objs:
-                    if isinstance(obj, self.model):
-                        if not router.allow_relation(obj, self.instance):
-                            raise ValueError(
-                                'Cannot add "%r": instance is on database "%s", value is on database "%s"' %
-                                (obj, self.instance._state.db, obj._state.db)
-                            )
-                        fk_val = self.through._meta.get_field(
-                            target_field_name).get_foreign_related_value(obj)[0]
-                        if fk_val is None:
-                            raise ValueError(
-                                'Cannot add "%r": the value for field "%s" is None' %
-                                (obj, target_field_name)
-                            )
-                        new_ids.add(fk_val)
-                    elif isinstance(obj, Model):
-                        raise TypeError(
-                            "'%s' instance expected, got %r" %
-                            (self.model._meta.object_name, obj)
-                        )
-                    else:
-                        new_ids.add(obj)
-
                 db = router.db_for_write(self.through, instance=self.instance)
-                vals = (self.through._default_manager.using(db)
-                        .values_list(target_field_name, flat=True)
-                        .filter(**{
-                            source_field_name: self.related_val[0],
-                            '%s__in' % target_field_name: new_ids,
-                        }))
-                new_ids.difference_update(vals)
+                missing_target_ids = self._get_missing_target_ids(source_field_name, target_field_name, db, objs)
 
                 with transaction.atomic(using=db, savepoint=False):
                     if self.reverse or source_field_name == self.source_field_name:
@@ -1100,16 +1107,16 @@ def create_forward_many_to_many_manager(superclass, rel, reverse):
                         signals.m2m_changed.send(
                             sender=self.through, action='pre_add',
                             instance=self.instance, reverse=self.reverse,
-                            model=self.model, pk_set=new_ids, using=db,
+                            model=self.model, pk_set=missing_target_ids, using=db,
                         )
 
                     # Add the ones that aren't there already
                     self.through._default_manager.using(db).bulk_create([
                         self.through(**through_defaults, **{
                             '%s_id' % source_field_name: self.related_val[0],
-                            '%s_id' % target_field_name: obj_id,
+                            '%s_id' % target_field_name: target_id,
                         })
-                        for obj_id in new_ids
+                        for target_id in missing_target_ids
                     ])
 
                     if self.reverse or source_field_name == self.source_field_name:
@@ -1118,7 +1125,7 @@ def create_forward_many_to_many_manager(superclass, rel, reverse):
                         signals.m2m_changed.send(
                             sender=self.through, action='post_add',
                             instance=self.instance, reverse=self.reverse,
-                            model=self.model, pk_set=new_ids, using=db,
+                            model=self.model, pk_set=missing_target_ids, using=db,
                         )
 
         def _remove_items(self, source_field_name, target_field_name, *objs):