'Return when the supplied field was changed.
Arguments:
key (`DjangoKeyValueStore.Key`): The field value to delete
Returns: datetime if there was a modified date, or None otherwise'

@contract(key=DjangoKeyValueStore.Key, returns='datetime|None')
def last_modified(self, key):

    if ((key.scope.user == UserScope.ONE) and (not self.user.is_anonymous())):
        assert (key.user_id == self.user.id)
    if (key.scope not in self.cache):
        return None
    return self.cache[key.scope].last_modified(key)
