node.js - Mongoose population on field other than _id (eg. String name) -

i trying populate reference field not correspond id of model referencing.

for example code

var locationschema = new schema({     name: {type: string, default: "location"},     location_id: {type: number, default: 0},     drones: [{ type: string, ref: 'drone' }], }); 

locations contain array of drone names, names unique.

i setup location , push strings onto drones array.

location.drones.push('test4'); location.save(); 

when go populate

location. findone({}). populate('drones', 'name'). 

i error

"message": "cast objectid failed value \"test4\" @ path \"_id\" model \"drone\"", 

my question how populate field such (string) name, instead of _id?