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