title: Laravel hasManyThrough - save doesn't work
body: <pre><code>table groups
  (
      id int auto_increment primary key,
      ...
  );
  
  table users
  (
      id int auto_increment primary key,
      ...
  );
  
  table user_groups
  (
      id int auto_increment primary key,
      user_id int not null,
      group_id int not null,
      ...
  );
  </code></pre>
  
  <p>In my Group model, i've a hasManyThrough relation to access users through user_groups linking table.</p>
  
  <pre><code>class Group
  {
      function users()
      {
          return $this-&gt;hasManyThrough('\App\Models\User', '\App\Models\UserGroup', 'group_id', 'id', 'id', 'user_id');
      }
  }
  </code></pre>
  
  <p>In my controller :</p>
  
  <pre><code>$user = User::find(2);
  $group-&gt;users[] = $user;
  $group-&gt;save();
  </code></pre>
  
  <p>So <code>$group-&gt;users</code> is a collection and i can add my user but it's not saving.</p>
  
  <p>How to add record on a <code>hasManyThrough</code> relation ?</p>