car.js

App.Car = SC.Record.extend({
name: SC.Record.attr(String),
  dealership: SC.Record.attr(App.Dealership),
  
  // For some reason we need this computed property to make SC.Query work
  dealershipGuid: function() {
    return this.get('dealership').get('guid');
  }.property('dealership').cacheable()
}) ;

dealership.js

App.Dealership = SC.Record.extend({
  name: SC.Record.attr(String),
  
  cars: function() {
    var q;
    
    q = SC.Query.create({
      recordType: App.Cars,
      conditions: "dealershipGuid = '" + this.get('guid') + "'"
    });
    
    return App.store.findAll(q);
  }.property()
  
  // memberships: SC.Record.toMany("App.Car", {
  //   inverse: 'dealership',
  //   isMaster: NO,
  //   
  // })
      // toMany fails so we have to build our own computed property
});