'Return ItemTagAssociation object for a user, item, and tag string; returns None if there is
no such association.'

def _get_item_tag_assoc(self, user, item, tag_name):

    scrubbed_tag_name = self._scrub_tag_name(tag_name)
    for item_tag_assoc in item.tags:
        if ((item_tag_assoc.user == user) and (item_tag_assoc.user_tname == scrubbed_tag_name)):
            return item_tag_assoc
    return None
