Access Related Model

Consider the following simple models...

//models/invoices.js
export default DS.Model.extend({
    created_by: DS.attr('number'),   //points to a user.id
    approved_by: DS.attr('number'),  //also points to a user.id
});

//models/users.js
export default DS.Model.extend({
    name: DS.attr('string')
});

Showing a list of invoices with the users's name field instead of their number:

# | Created | Approved
--------------------------
2 | Jim     | Bobby  
3 | Sue     | Betty

My question: How can I also retrieve the user's name when I only have access to their id in the columns: created_by and for approved_by?