mongodb - perform batch query

Question

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

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?

Answer 1

You'll need to do it as two steps.

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] }})