magic-removal: Fixed use of return value of cursor.execute() in m2m add() and added a test to make sure

git-svn-id: http://code.djangoproject.com/svn/django/branches/magic-removal@2289 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/fields/related.py

@@ -123,11 +123,14 @@ def _add_m2m_items(rel_manager_inst, managerclass, rel_model, join_table, this_c
     # First find out which items are already added, to avoid adding them twice
     new_ids = set([obj._get_pk_val() for obj in objs])
     cursor = connection.cursor()
-    rowcount = cursor.execute("SELECT %s FROM %s WHERE %s = %%s AND %s IN (%s)" % \
+    cursor.execute("SELECT %s FROM %s WHERE %s = %%s AND %s IN (%s)" % \
         (rel_col_name, join_table, this_col_name,
         rel_col_name, ",".join(['%s'] * len(new_ids))), 
         [this_pk_val] + list(new_ids))
-    existing_ids = set([row[0] for row in cursor.fetchmany(rowcount)])
+    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()
 
     # Add the ones that aren't there already
     for obj_id in (new_ids - existing_ids):

tests/modeltests/many_to_many/models.py

@@ -44,6 +44,9 @@ API_TESTS = """
 >>> a2.publications.add(p1, p2)
 >>> a2.publications.add(p3)
 
+# Adding a second time is OK
+>>> a2.publications.add(p3)
+
 # Add a Publication directly via publications.add by using keyword arguments.
 >>> a2.publications.add(title='Highlights for Children')