本文介绍了Mongodb-$ set和$ setOnInsert中的重复字段的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
在 帖子，接受的答案说明您在upsert操作中的$set和$setOnInsert下不能有相同的字段.
In this post, the accepted answer explains that you cannot have the same fields under $set and $setOnInsert in an upsert operation.
有人可以解释为什么会这样吗?似乎$setOnInsert不应与$set冲突，因为前者在插入文档时使用，而后者在更新文档时使用.
Can someone explain why this is? It seems like the $setOnInsert shouldn't conflict with $set, since the former is used when a document is inserted, and the latter is used when the document is updated.
推荐答案
$ set 运算符也用于 upsert .因此，在 $ set 和 $ setOnInsert 上引用相同的字段是没有意义的.
$set operator is used on upsert too. So it's nonsense to refer same fields both on $set and $setOnInsert.
只需在一个空集合上尝试一下:
Just try this on an empty collection:
db.items.remove();
db.items.update({},{$set:{a:1},$setOnInsert:{b:2}},{upsert:1})
db.items.find({});
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