@classmethod
def get_temporary_identifier(cls, store):
    """Returns a temporary negative identifier

        This should be used when working with syncronized databases and a
        purchase order is being created in a branch different than the
        destination branch.

        The sincronizer will be responsible for setting the definitive
        identifier once the order arives at the destination
        """
    lower_value = store.find(cls).min(cls.identifier)
    return min(lower_value or 0, 0) - 1
