'Returns user\'s rating for an item. Return type is <item_class>RatingAssociation.'

def get_user_item_rating(self, db_session, user, item, webapp_model=None):

    if (webapp_model is None):
        webapp_model = galaxy.model
    item_rating_assoc_class = self._get_item_rating_assoc_class(item, webapp_model=webapp_model)
    if (not item_rating_assoc_class):
        raise RuntimeException(('Item    does    not    have    ratings:    %s' % item.__class__.__name__))
    item_id_filter = self._get_item_id_filter_str(item, item_rating_assoc_class)
    return db_session.query(item_rating_assoc_class).filter_by(user=user).filter(item_id_filter).first()
