title: Get belongsTo ID without fetching record
body: <p>I'm trying to fetch the belongsTo ID without fetching the actual record. My JSON API returns the ID for the belongsTo relation.</p>
  
  <p><strong>I have the following models</strong></p>
  
  <pre><code>App.Recipe = DS.Model.extend(
    title: DS.attr()
    ingredients: DS.hasMany('ingredient', async: true)
  )
  
  App.Ingredient = DS.Model.extend(
    recipe: DS.belongsTo('recipe')
    product: DS.belongsTo('product', async: true)
  )
  
  App.Product = DS.Model.extend(
    title: DS.attr()
  )
  </code></pre>
  
  <p><strong>This is my route:</strong></p>
  
  <pre><code>App.RecipeIndexRoute = Ember.Route.extend(
    model: (args) -&gt;
      @store.find('recipe', args.recipe_id)
  )
  </code></pre>
  
  <p><strong>This is my controller:</strong></p>
  
  <p>I'm trying to find the product id within this controller.</p>
  
  <pre><code>App.RecipeIndexController = Ember.ObjectController.extend(
    hasRecipeInCart: null
  
    actions:
      toggleCart: -&gt;
        @content.get('ingredients').then((ingredients) =&gt;
          # how do I get product id here, without lookup up the product relation?
          # this 'get' makes Ember call: /api/v1/products/1
          # I simply want the product ID (in this case 1), without having to call the server again.
          ingredient.get('product').then((product) =&gt;
            product.id # =&gt; 1
          )
  </code></pre>
  
  <p><strong>My JSON looks like this:</strong></p>
  
  <p>HTTP GET: /api/v1/recipes/1</p>
  
  <pre><code>{
    "recipe": {
      "id":1,
      "title":"Recipe 1",
      "ingredients":[2,1]
    }
  }
  </code></pre>
  
  <p>HTTP GET: /api/v1/ingredients?ids[]=2&amp;ids[]=1</p>
  
  <pre><code>{
    "ingredients": [
      {
        "id":2,
        "recipe":1,
        "product":1
      },
      {
        "id":1,
        "recipe":1,
        "product":2
      }
    ]
  }
  </code></pre>