MongoDB Shell: How to Update a Collection from a Collection

Question

I have two collections: links and children.

Links has values like this:

> db.links.find().pretty()
{ "_id" : ObjectId("567374999df36aeeda6f2a5f"), "EID" : 1, "CID" : 1 }

{ "_id" : ObjectId("567374999df36aeeda6f2a60"), "EID" : 1, "CID" : 3 }

{ "_id" : ObjectId("567374999df36aeeda6f2a61"), "EID" : 2, "CID" : 5 }

Children has values like this:

{
        "_id" : ObjectId("567382709df36aeeda6f2a7e"),
        "CID" : 2,
        "Cname" : "Mo",
        "Age" : 11
}
{
        "_id" : ObjectId("567382709df36aeeda6f2a7f"),
        "CID" : 3,
        "Cname" : "Adam",
        "Age" : 13
}
{
        "_id" : ObjectId("567382709df36aeeda6f2a80"),
        "CID" : 4,
        "Cname" : "Eve",
        "Age" : 21
}

It seems like this should add EID to children where CID matches between links and children, but it does nothing.

db.children.find().forEach(function (doc1) {
    var doc2 = db.links.find({ id: doc1.CID });
    if (doc2.CID == doc1.CID) {
        doc1.EID = doc2.EID;
        db.children.save(doc1);
    }
});

This sets the value of EID in children to the value of EID from links where the value of CID in children == 14.

db.children.find().forEach(function (doc1) {
    var doc2 = db.links.find({},{_id: 0, CID: 1});
    if (doc1.CID == 14) {
        doc1.EID = doc1.CID;
        db.children.save(doc1);
    }
});

{
        "_id" : ObjectId("567382709df36aeeda6f2a8a"),
        "CID" : 14,
        "Cname" : "George II",
        "Age" : 12,
        "EID" : 14
}

So, it seems like that the equality operation in Javascript (doc2.CID == doc1.CID) isn't working. Why won't this work? I assume I have the syntax wrong or am using the wrong equality operation or operator?

Answer 1

var doc2 = db.links.find({ id: doc1.CID });

doc2 here is a cursor, not the document itself. So you need to go through documents in the cursor.

Try the following code:

db.children.find().forEach(function (doc1) {     
    var doc2 = db.links.find({CID:doc1.CID}).forEach(function(doc2){
        doc1.EID = doc2.EID; 
        db.children.save(doc1);
    })
})