How to update a collection based on another collection in MongoDB?

Question

Now I get two collections: coll01 and coll02 .

And the structure of coll01 is like this:

{
  id: 01,
  name: "xxx",
  age: 30
}

and the structure of coll02 is like:

{
  id: 01,
  name: "XYZ"
  gender: "male"
}

The two id fields in the both collection are indices. And the numbers of documents in these two collections are same.

And what I want to do in traditional SQL is :

update coll01, coll02
set coll01.name = coll02.name
where coll01.id = coll02.id

Answer 1

Mongodb is not relational database and doesn't support join's. So, you should think a little, do you really need mongodb for your purposes?

Solution for update: you can update each document from coll01 in loop:

db.coll01.find().forEach(function (doc1) {
    var doc2 = db.coll02.findOne({ id: doc1.id }, { name: 1 });
    if (doc2 != null) {
        doc1.name = doc2.name;
        db.coll01.save(doc1);
    }
});

Index for id field in coll02 collection will decrease find() operation execution time inside the loop. Also, see about server-side JavaScript execution: Running .js files via a mongo shell Instance on the Server