'Returns
self.user_preferences :  list [(item_id,preference)]
Return user\'s preferences, ordered by user ID (if order_by_id is True)
or by the preference values (if order_by_id is False), as an array.'

def preferences_from_user(self, user_id, order_by_id=True):

    preferences = self.preference_values_from_user(user_id)
    preferences = preferences.flatten()
    return self._item_ids[preferences]
