Tweak relationship for Department._super_parents

since there can be more than one

corepos/db/office_op/model.py

@@ -100,7 +100,9 @@ class SuperDepartment(Base):
         doc="""
         Reference to the child department for this mapping.
         """,
-        backref=orm.backref('_super_parent', uselist=False))
+        backref=orm.backref(
+            '_super_parents',
+            order_by=parent_id))
 
     def __str__(self):
         return "{} / {}".format(self.parent, self.child)