Mongoose get _id of referenced model, no matter if it is populated or not

Given this Category Schema

{
    title: String,
    parent: {
        type: Schema.Types.ObjectId,
        ref: 'Category',
        required: true
    }
}

Now what if I have an instance of Category, but I don't know if parent has been populated or not and I need the _id (actually just the hex representation) of the parent.

Is there a nicer way of doing this: ?

var parentID = (category.parent._id || category.parent).toString();