mongodb-执行批量查询 [英] mongodb - perform batch query

问题描述

我首先需要从集合b中查询数据，然后根据这些数据从集合b中查询数据.如:

I need query data from collection a first, then according to those data, query from collection b. Such as:

For each id queried from a
    query data from b where "_id" == id

在SQL中，这可以通过联接表a& b一次选择.但是在mongodb中，它需要执行多次查询，效率似乎低下，不是吗?还是可以仅通过2个查询来完成?(一个用于a，另一个用于b，而不是1加n)我知道NoSQL不支持联接，但是有一种方法可以将for循环中的查询批量执行到单个查询中. ?

In SQL, this can be done by join table a & b in a single select. But in mongodb, it needs do multi query, it seems inefficient, doesn't it? Or it can be done by just 2 queries?(one for a, another for b, rather than 1 plus n) I know NoSQL doesn't support join, but is there a way to batch execute queries in for loop into a single query?

推荐答案

您需要分两个步骤进行操作.

You'll need to do it as two steps.

查看$in运算符(参考)，例如，允许传递_id的数组.许多人建议您批量执行1000次.

Look into the $in operator (reference) which allows passing an array of _ids for example. Many would suggest you do those in batches of, say, 1000 _ids.

db.myCollection.find({ _id : { $in : [ 1, 2, 3, 4] }})

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