Get single record from collection in ember-cli-mirage

How can I return a record that matches some custom id (not the regular id) from my collection?

// record schema
{
   id: 123,           // assigned id from mirage
   dId: 'DEVICE0001', // device id I want to use to pick
   value: 'some content'
}   

To select with something like this:

// app/mirage/config.js
this.get("/something/:device_did", function(db, request){
        var mid = request.params.device_did;

        // select using my device id
        return db.devices.firstWhere( { 'dId': did });
    });

The API docs only reference a 'find()' function that acts on the id, and a 'where()' that gets an array of records.