'Returns
self
Sets a particular preference (item plus rating) for a user.'

def set_preference(self, user_id, item_id, value=None):

    user_id_loc = np.where((self._user_ids == user_id))
    if (not user_id_loc[0].size):
        raise UserNotFoundError('user_id    in    the    model    not    found')
    self.dataset[user_id].append(item_id)
    self.build_model()
