gistfile1.txt

App.House = DS.Model.extend({
  doors: DS.hasMany('door');
});

App.Door = DS.Model.extend({
  index: DS.attr();  //this doors position in the list of Many doors
  house: DS.belongsTo('house');
});

App.DoorController = Ember.ObjectController.extend({
  previous_door_id: function() {
    var model = this.get('model');
    var index = model.get('index');
    var house = model.get('house');
    var doors = house.get('doors');
    var previous_door = doors[index-1];
    return previous_door.id;
  }.property('id')
});