MongoDB can't update document because _id is string, not ObjectId

Question

I'm doing a rest api to exchange data between a mongo database and a web app. These data are json formatted.

I have a trouble when it comes to update a document:

cannot change _id of a document.

As a matter of fact, in my JSON the _id of the doc is stored as a string and deserialized as a string. Whereas it is stored as an ObjectID in mongo. This explains why mongo raises an error.

In mongo: _id: ObjectId('51051fd25b442a5849000001')
In JSON: _id:"51051fd25b442a5849000001"

To avoid this I manually convert the _id property from a string to an ObjectID. But It seems ugly and will fail with other BSON types.

Q: Is there a clean way to avoid that or to do a nice JSON/BSON conversion?

Below is the code I use to update a document. I'm using nodejs with express and mongodb with the native driver.

exports.updateById = function(req, res) {
var id = req.params.id;
var map = req.body;

map._id = new ObjectID.createFromHexString( map._id); // Manual conversion. How to avoid this???

console.log( 'Updating map: ' + id);
console.log( 'Map: ' + JSON.stringify( map));

db.collection('maps', function(err, collection) {
    if(err) throw err;
    collection.update(
        {'_id': new BSON.ObjectID(id)}, map, {safe:true}, 
        function(err, result) {
            if (err) {
                console.log('Updating map err: ' + JSON.stringify( err));
                res.json( 500, {'message':'An error has occurred while updating the map', 'error': err});
            } else {
                console.log('Updating succeed');
                res.send(map);
            }
        }
    );
});

};

JohnnyHK JohnnyHK · Accepted Answer · 2013-01-29T14:50:16

Because you can't modify the _id field, a better approach is to simply remove the that field from your map object instead of converting it to an ObjectId.

So this:

delete map._id;

instead of this:

map._id = new ObjectID.createFromHexString( map._id);

If you want to return the updated object like you're attempting with res.send(map);, you should be using findAndModify instead of update so you have access to the resulting doc and not just what was posted.

MongoDB can't update document because _id is string, not ObjectId

1 Answers