'Changing the tags on a document removes the old tags from
that document.'

def test_edit_document_POST_removes_old_tags(self):

    self.d.current_revision = None
    self.d.save()
    topics = [TopicFactory(), TopicFactory(), TopicFactory()]
    self.d.topics.add(*topics)
    eq_(self.d.topics.count(), len(topics))
    new_topics = [topics[0], TopicFactory()]
    data = new_document_data((t.id for t in new_topics))
    data['form'] = 'doc'
    self.client.post(reverse('wiki.edit_document', args=[self.d.slug]), data)
    topic_ids = self.d.topics.values_list('id', flat=True)
    eq_(2, len(topic_ids))
    assert (new_topics[0].id in topic_ids)
    assert (new_topics[1].id in topic_ids)
