Python 如何使引用字段的列表字段唯一？

我试图使

ReferenceField

的

ListField

唯一，但我得到了一个唯一的错误，而我不应该这样做。为什么呢

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()

NotUniqueer错误：尝试保存重复的唯一密钥（E11000重复密钥错误集合：test_db.training index:transactions_1 dup key:{:ObjectId（'575D33ECEEFEE54280475FC1B'））