How to do inner reference with mongoose?

How to do inner reference with mongoose?

I want to do something like following code, but it failed.
var User = new Schema({
name: { type: String, required: true },
phone_number: { type: String, required: true },
modified: { type: Date, default: Date.now },
contacts: [{
user: { type : Schema.ObjectId, ref : 'User' }
}]
});
var UserModel = mongoose.model('User', User);
Is it able to achieve that purpose?