仅当Dynamo的值相同或记录不存在时才添加项

Question

最近，我开始和迪纳摩合作。遇到一个场景，我只想在它已经不存在或者属性值相同的情况下添加条目。 例如

insert {'h': 1, 'r': 2, 'attr': {'attr': 'value1'}} should succeed, as record not present already
insert {'h': 1, 'r': 2, 'attr': {'attr': 'value1'}} should succeed, as same record exist with same information
insert {'h': 1, 'r': 2, 'attr': {'attr': 'value2'}} should fail, as attribute value is different

由于我想在单个指令中保持原子性，如何在Dynamo中使用期望值条件来实现这一点？你知道吗

有什么帮助吗