How can I set a string from variable as a key in existing object in mongodb

Question

I know that this is working:

db.users.update({_id: "1234"}, {$set: {"conversations.partId": id}});

In result I've got: conversations:{"partId":2345}

but I would like to set partId as a variable. And to have structure like this: conversations:{"3456":"2345"}

I have tried:

db.users.update({_id: "someid"}, {$set: {"conversations".partId: id}});
or
db.users.update({_id: "someid"}, {$set: {"conversations"+partId: id}});

But this is of course not working

Can anyone help me?

chridam · Accepted Answer · 2015-09-03 13:10:15Z

1

Try creating the document you need to update via the bracket notation, i.e.

var obj = {};
obj[partId] = id;

You can then use that as a new embedded document to add to the conversations field with the $set operator expression as follows:

db.users.update(
    {_id: "someid"}, 
    {
        $set: {
            "conversations": obj
        }
    }
);

answered Sep 3, 2015 at 13:10

chridam

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2 Comments

mwiacko Over a year ago

thank you this is working. But is there a better way of adding another key:value than fetching from database existing key:value's, adding them to that obj and $set'ing conversation object from start? I'm thinking about something like $addToSet or $push but for objects

chridam Over a year ago

If you are updating the existing schema to have the key:value relationship then the only way is updating the embedded documents using a loop and if you are using MongoDB versions > 2.6 then bulk operations can improve the performance. Other than that I don't see any other better way, and for the record, $addToSet or $push operators work on arrays.

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