如何使ReferenceFields的ListField唯一？

Question

我正在努力使ListField ReferenceField唯一，但我不应该得到一个独特的错误。这是为什么？

class Training(Document):

    transactions = ListField(ReferenceField(Buy, reverse_delete_rule=PULL), default=list, unique=True)

b = Buy()
b.save()
b1 = Buy()
b1.save()
b2 = Buy()
b2.save()

# both should save fine since the lists are different
Training(transactions=[b, b1]).save()

Training(transactions=[b, b2]).save()

NotUniqueError：尝试保存重复的唯一键（E11000重复键错误集合：test_db.training index：transactions_1 dup key：{：ObjectId（'575d33eceefe54280475fc1b'）}）