How does javascript forEach() function iterate documents in mongodb shell?

Question

How does javascript forEach() function work in mongodb shell?

With what sequence are the documents being processed?

Eg, let's say that I have two big collections and those two collections should contain the same number of documents (note that the structure of the document isn't necessarily the same), and I want to iterate the first and see if for each document in the first collection, there's a record for it in the second collection :

db.MyColl.find().forEach(
    function(aDocument) { 
        print("Checking :"+aDocument._MyId)
        var db2 = db.getSiblingDB("mySiblingDb");
        var aDocument2 = db2.MyColl.findOne({_MyId: aDocument._MyId});    
        if (aDocument2 == null) { 
           print("Missing document with _MyId: " + aDocument._MyId);
        }  
    }
);

Is this piece of code going to iterate the documents with the same sequence?

What do you mean by "iterate the documents with the same sequence"? Your code looks like it does a `find()` on one collection (collection `A`), then tries to `findOne()` the corresponding matching document on the second collection (collection `B`). There is only one iteration in the code, and that is inside the `forEach()`. I guess I'm not understanding your intent here. Are you asking about the ordering of the sequence that will be processed by the `forEach()`? — kevinadi, Jul 27 '18 at 00:08

score 0 · Answer 1 · answered Jul 26 '18 at 08:19

0

db.collection.find(query).forEach(function(err, doc) {
  // ...
});

Foreach return error function as first and document as second argument

answered Jul 26 '18 at 08:19

thatcoder

367
2
9

1

Not necessarily. If you provide one argument in the callback, it's your item and not the error message. – Dimitrios Jul 26 '18 at 08:23
http://fredkschott.com/post/2014/03/understanding-error-first-callbacks-in-node-js/ – thatcoder Jul 26 '18 at 08:25
2

The OP is referring to mongo shell operations, not node.js – chridam Jul 26 '18 at 08:27

How does javascript forEach() function iterate documents in mongodb shell?

1 Answers1