title: Eloquent where clause on belongsTo relationship
body: <p>Lets say I have already loaded the relationship and now I want to get models that fulfill it.<br>
  Is there a way to do it in an objective way, rather than using ids directly?<br>
  Here is an example Controller describing what I want to achieve:</p>
  
  <pre><code>public function videos(User $user, Category $category)
  {
      $videos = Video::where('user', $user)-&gt;where('category', $category)-&gt;get();
      // ...
  }
  </code></pre>
  
  <p>I know I can do it like this, but I would like to skip extracting the id and leave it to the framework if possible:</p>
  
  <pre><code>public function videos(User $user, Category $category)
  {
      $videos = Video::where('user_id', $user-&gt;id)-&gt;where('category_id', $category-&gt;id)-&gt;get();
      // ...
  }
  </code></pre>