diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index fad9c164c1..c336585ac9 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -335,10 +335,7 @@ def create_many_related_manager(superclass):
                     (target_col_name, self.join_table, source_col_name,
                     target_col_name, ",".join(['%s'] * len(new_ids))),
                     [self._pk_val] + list(new_ids))
-                if cursor.rowcount is not None and cursor.rowcount != 0:
-                    existing_ids = set([row[0] for row in cursor.fetchmany(cursor.rowcount)])
-                else:
-                    existing_ids = set()
+                existing_ids = set([row[0] for row in cursor.fetchall()])
 
                 # Add the ones that aren't there already
                 for obj_id in (new_ids - existing_ids):