Mongoose populate limit 的正确用法

Mongoose 作为 Nodejs 为 MongoDB 构建 Model 层的一个库,使用较为方便,其中的 Ref 与传统 SQL 中的外键较为类似,在 query 时,只需加上 populate 即可将相关的字段查找出来。

但其中有一项需要注意的是:

Populate does support a limit option, however, it currently does not limit on a per-document basis. For example, suppose you have 2 stories:

If you were to populate() using the limit option, you would find that the 2nd story has 0 fans:

That's because, in order to avoid executing a separate query for each document, Mongoose instead queries for fans using numDocuments * limit as the limit. If you need the correct limit, you should use the perDocumentLimit option (new in Mongoose 5.9.0). Just keep in mind that populate() will execute a separate query for each story.

若想约束每个“外键”返回的数量,则不能使用 limit,如果用 limit 则会出现后续的结果中并没有返回值,若想严格约束,则需使用 perDocumentLimit.

https://thecodebarbarian.com/whats-new-in-mongoose-59-schematype-default-options-and-per-document-populate.html

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