@classmethod
def get_current(cls, app, tag):
    """
        Given an app and a tag, look for a build that matches both, and was
        successfully built (or is currently building).  If not found, return
        None.
        """
    builds = cls.objects.filter(app=app, tag=tag).exclude(status='expired'
        ).exclude(status='failed').order_by('-id')
    if not builds:
        return None
    return builds[0]
