def _create_id(data_set_id, record_id):
    """
    The record_ids are not necessarily globally unique, because they're user provided and
    are only guaranteed to be unique within a collection. So that we have a unique primary
    key we're joining them with a colon.

    >>> _create_id('foo', 'bar')
    'foo:bar'
    """
    return ':'.join([data_set_id, record_id])