title: Laravel Model binding many to many realtionship does not populate data
body: <p>I have two tables with a many to many relation (Project and Center, the pivot table is ProjectCenter).</p>
  
  <p>These are my models:</p>
  
  <p>Project:</p>
  
  <pre><code>class Project extends Model {
      public function centers()
      {
          return $this-&gt;belongsToMany('App\Models\Center', 'ProjectCenter', 'IDProject', 'IDCenter');
      }
      public function getCenterListAttribute()
      {
          return $this-&gt;centers-&gt;lists('IDCenter')-&gt;all();
      }
  }
  </code></pre>
  
  <p>Center:</p>
  
  <pre><code>class Center extends Model {
      public function projects()
      {
          return $this-&gt;belongsToMany('App\Models\Project', 'ProjectCenter', 'IDCenter', 'IDProject');
      }
  }
  </code></pre>
  
  <p>Controller -> edit:</p>
  
  <pre><code>public function edit($id)
      {
          $project = Project::find($id);
          $centerList = Center::lists('Name', 'IDCenter')-&gt;toArray();
          return view('project/add', array('centerList' =&gt; $centerList))-&gt;with('project', $project);
      }
  </code></pre>
  
  <p>And the view:</p>
  
  <pre><code>{!! Form::label('centers_list', 'Center*') !!}
  {!! Form::select('centers_list[]', 
      $centerList, 
      null, 
      array(
          'class' =&gt; 'form-control ', 
          'required' =&gt; 'required', 
          'multiple' =&gt; true, 
          'data-placeholder' =&gt; 
          'Select a center'
     )
   ) !!}
  </code></pre>
  
  <p>But I can not select the data already stored previously.</p>
  
  <p>For example: the project 8 (IDProject) has two centers (1 and 2) but the data is not populated in the multiple select:</p>
  
  <p><a href="https://i.stack.imgur.com/ckwTF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ckwTF.png" alt="enter image description here"></a></p>
  
  <p>What am I doing wrong?</p>