title: Laravel get all the models with relations
body: <p>I can't find how to get all of the elements with relations to another element using Eloquent in Laravel.
  I have Substances and Contents and want all the contents related to the substances with name 'subst'.</p>
  
  <p>My relation Substance - Content:</p>
  
  <pre><code>public function relation () 
  {
      return $this-&gt;belongsToMany('Substance', 'contents_substances', 'id_contents', 'id_substances');
  }
  
  public function relation () 
  {
      return $this-&gt;belongsToMany('Content', 'contents_substances', 'id_contents', 'id_substances');
  }
  </code></pre>
  
  <p>I know I can get all the contents related to one substance:</p>
  
  <pre><code>$content = Substance::find(1)-&gt;relation()-&gt;get();
  </code></pre>
  
  <p>but is possible to get all the contents related to a group of substances?</p>
  
  <p>something like:</p>
  
  <pre><code>$sub = Substance::where('name', '=', 'subst'); // get all the substances with that name
  $contents =$sub-&gt;relation()-&gt;get(); // ???
  </code></pre>
  
  <p>Any help is appreciated!</p>