title: Ember.js : How to find records that do NOT have a property set?
body: <p>I have a bunch of Records that "belongsTo" other records. (Different models, just typical 1-n relationship.) How can I call find on my model to find all those that have NO belongs-to association?</p>
  
  <p>Something like:</p>
  
  <pre><code>App.Thing = DS.Model.extend({
   other: DS.belongsTo( 'App.Other' ),
   someattr: DS.attr('string')
  });
  
  App.Thing.find({ other: null });
  </code></pre>
  
  <p><strong>EDIT:</strong></p>
  
  <p>louiscoquio's solution didn't work for me. So I thought I could filter in the controller.</p>
  
  <p>Here's how I try to do it:</p>
  
  <pre><code>App.ThingsAsideController = Ember.ArrayController.extend({
    unassigned: function() {
     return this.filterProperty('other');
    }.property('content.@each')
  });
  </code></pre>
  
  <p>This works just fine if I filter for <code>someattr</code> but I cannot filter for the relationship.</p>
  
  <p>I guess the question is: What is the property name for the belongsTo association?</p>
  
  <p><strong>EDIT</strong>, SOMETHING THAT WORKS:</p>
  
  <p>This works, but I'm very unhappy with it:</p>
  
  <pre><code> App.ThingsAsideController = Ember.ArrayController.extend({
     unassigned: function() {
       return this.filter(function(item, index, enumerable){
  
         var belongsToOther = false;
  
         var otherThings = App.OtherThing.find();
         otherThings.forEach( function( otherThing ){
           otherThing.get('things').forEach( function( otherThingThing ) {
              if( otherThingThing.id === item.id ) {
                belongsToOther = true;
              }
           });
         });
  
         return !belongsToOther;
      });
  </code></pre>
  
  <p>I've been reading ember-data source but I can't find out how to query the belongs-to association of a record.</p>