Mongodb - matching a nested document with an unknown key

Question

Within a collection I am storing some data, mapped against a mobile device UID which is generated elsewhere. For example:

{
    devices: {
        'b2e4fe52d4ab57fd55fa': { model: 'aPhone', number: 1111111 },
        'b2e4fe52d4ab57fd55fb': { model: 'bPhone', number: 2222222 },
        'b2e4fe52d4ab57fd55fc': { model: 'cPhone', number: 3333333 }
    }
}

Is there a way to retrieve the object representing one device without using the UID?

This would be easy for an array simply using an $elemMatch on either the model or number, but it seems it's use is limited only Arrays, is there an analogue operator for Documents?

Answer 1

is there an analogue operator for Documents

No. There is no operator to match and project a single key value pair inside a document, without knowing the key.

You will have to write the code to do the post processing on each devices document, which may look like:

var devices = db.devices.findOne().devices; 
    var keys = Object.keys(devices);
    for(var i=0;i<keys.length;i++){
    var device = devices[keys[i]];
    if(device.model == 'aPhone' && device.number == 1111111)
    {
        print(keys[i]);
        // do the processing here
        break;
    }
}

In case you had an option to change your schema as below, which is much more flexible and looks logically correct too,

db.devices.insert({
    devices: [
        {"id":"b2e4fe52d4ab57fd55fa",  model: 'aPhone', number: 1111111 },
        {"id":"b2e4fe52d4ab57fd55fb",  model: 'bPhone', number: 2222222 },
        {"id":"b2e4fe52d4ab57fd55fc",  model: 'cPhone', number: 3333333 }
    ]
})

Then you have the luxury of achieving it with a simple find and project query.

db.devices.findOne({"devices.model":"aPhone",
                    "devices.number":1111111},
                   {"devices.$":1})