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?
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] }})
It's very simple, don't make so many DB calls for each id, it is very inefficient, it is possible to execute a single query which will return all documents relevant to each of the ids in a single pass using the $in operator in MongoDB, which is synonymous to in syntax in SQL so for example if you need to find out the documents for 5 ids in a single pass then
const ids = ['id1', 'id2', 'id3', 'id4', 'id5'];
const results = db.collectionName.find({ _id : { $in : ids }})
This will get you all the relevant documents in a single pass.
