MongoDB原子"findOrCreate":findOne，如果不存在则插入，但不更新 [英] MongoDB atomic "findOrCreate": findOne, insert if nonexistent, but do not update

问题描述

如标题所示，我想通过_id对文档执行查找(一个)，如果不存在，则创建它，然后查找它是否被创建，是否在回调中返回它

as the title says, I want to perform a find (one) for a document, by _id, and if doesn't exist, have it created, then whether it was found or was created, have it returned in the callback.

我不想更新它(如果存在的话)，就像我读过的findAndModify一样.我在Stackoverflow上看到了许多其他与此相关的问题，但再次，我不想更新任何内容.

I don't want to update it if it exists, as I've read findAndModify does. I have seen many other questions on Stackoverflow regarding this but again, don't wish to update anything.

我不确定是否通过创建(不存在)实际上是每个人都在谈论的更新，这一切都令人困惑:(

I am unsure if by creating (of not existing), THAT is actually the update everyone is talking about, it's all so confuzzling :(

推荐答案

从MongoDB 2.4开始，对于像findOrCreate这样的原子操作，不再需要依赖唯一索引(或任何其他解决方法).

Beginning with MongoDB 2.4, it's no longer necessary to rely on a unique index (or any other workaround) for atomic findOrCreate like operations.

这要感谢2.4新增的 $setOnInsert运算符.允许您指定仅在插入文档时发生的更新.

This is thanks to the $setOnInsert operator new to 2.4, which allows you to specify updates which should only happen when inserting documents.

这与upsert选项结合使用，意味着您可以使用findAndModify来实现类似于findOrCreate的原子操作.

This, combined with the upsert option, means you can use findAndModify to achieve an atomic findOrCreate-like operation.

db.collection.findAndModify({
  query: { _id: "some potentially existing id" },
  update: {
    $setOnInsert: { foo: "bar" }
  },
  new: true,   // return new doc if one is upserted
  upsert: true // insert the document if it does not exist
})

由于$setOnInsert仅影响插入的文档，因此，如果找到现有文档，则不会进行任何修改.如果不存在文档，它将使用指定的_id向上插入一个文档，然后执行仅插入集.在这两种情况下，文档都会被退回.

As $setOnInsert only affects documents being inserted, if an existing document is found, no modification will occur. If no document exists, it will upsert one with the specified _id, then perform the insert only set. In both cases, the document is returned.

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