title: createRecord with belongsTo association
body: <p>I'm trying to store a record with a belongsTo association, but the associated ID is always empty:</p>
  
  <p><strong>Customer Model</strong></p>
  
  <pre><code>Docket.Customer = DS.Model.extend({
    name:        DS.attr('string'),
    initial:     DS.attr('string'),
    description: DS.attr('string'),
    number:      DS.attr('string'),
    archived:    DS.attr('boolean'),
    projects:    DS.hasMany('project',{ async: true })
  });
  </code></pre>
  
  <p><strong>Project Model</strong></p>
  
  <pre><code>Docket.Project = DS.Model.extend({
    name:        DS.attr('string'),
    description: DS.attr('string'),
    number:      DS.attr('string'),
    archived:    DS.attr('boolean'),
    customer:    DS.belongsTo('customer')
  });
  </code></pre>
  
  <p><strong>Save action in ProjectsController</strong></p>
  
  <pre><code>var _this = this;
  
  // create new record
  var project = this.store.createRecord('project', this.getProperties('name','number', 'description', 'archived'));
  
  // add customer with id 22 to project
  this.store.find('customer', 22).then(function(customer) {
    project.set('customer', customer);
  });
  
  // save if validation passes, otherwise show errors
  project.save().then(function(){
    _this.resetAndCloseForm(form);
  }, function(response){
    _this.set('errors',response.errors);
  });
  </code></pre>
  
  <p>The json sended to server is always this:</p>
  
  <pre><code>archived: false
  customer_id: null
  description: null
  name: "foobar"
  number: null
  </code></pre>
  
  <p>I want to choose the customer of the project in a selectbox. Are there any smart ways to get the selected customer as record or do I have to load it over the ID?</p>
  
  <pre><code>{{view Ember.Select
      viewName="select"
      contentBinding="customers"
      optionLabelPath="content.name"
      optionValuePath="content.id"
      prompt="Pick a customer:"
      selectionBinding="selectedCustomer"
  }}
  </code></pre>