Returns a function that accepts an old_id and returns the new ID for the enclosed relation
    name.

def id_getter(self, relation_name, strict=False):
    """Returns a function that accepts an old_id and returns the new ID for the enclosed relation
    name."""
    def get_id(old_id):
      """Get the new ID for the enclosed relation, given an old ID."""
      return self.get_new_id(relation_name, old_id, strict)
    return get_id