Given a tree index, retrieves the ids atached to the given prefix,
        think of if as a mechanism for pattern suscription, where two models
        attached to the `a`, `a:b` respectively are found by the `a:b` string,
        because both model's subscription key matches the string.

def tree_match(cls, field, string):
        ''' Given a tree index, retrieves the ids atached to the given prefix,
        think of if as a mechanism for pattern suscription, where two models
        attached to the `a`, `a:b` respectively are found by the `a:b` string,
        because both model's subscription key matches the string. '''
        if not string:
            return set()

        redis = cls.get_redis()
        prefix = '{}:tree_{}'.format(cls.cls_key(), field)
        pieces = string.split(':')

        ans =  redis.sunion(
            prefix + ':' + ':'.join(pieces[0:i+1])
            for i in range(len(pieces))
        )

        return sorted(map(
            lambda id: cls.get(id),
            map(
                debyte_string,
                ans
            )
        ), key=lambda x:x.id)