'Return the list of topical parent documents above this one,
or an empty list if none exist.'

@property
def parents(self):

    if (self.parent_topic is None):
        return []
    current_parent = self.parent_topic
    parents = [current_parent]
    while (current_parent.parent_topic is not None):
        parents.insert(0, current_parent.parent_topic)
        current_parent = current_parent.parent_topic
    return parents
