title: Laravel 5.2 is not update parent's timestamp after updating child
body: <p>In Laravel framework, I have 2 models. First is question model:</p>
  
  <pre><code>class Question extends Model
  {
       protected $table = 'questions';
       protected $fillable = [...];
       public function answers()
       {
           return $this-&gt;hasMany('App\Models\Answer');
       }
       ...
  }
  </code></pre>
  
  <p>And Answer model:</p>
  
  <pre><code>class Answer extends Model
  {
       protected $table = 'answers';
       protected $fillable = [...];
       protected $touches = ['question'];
       public function question()
       {
           return $this-&gt;belongsTo('App\Models\Question');
       }
       ...
  }
  </code></pre>
  
  <p>I updated some attribute in answer model with code:</p>
  
  <pre><code>$answer = $answer-&gt;select('id')
              -&gt;where('id', $id)
              -&gt;firstOrFail();
  $answer-&gt;update($data);
  </code></pre>
  
  <p>When I check database, data in Answers table is updated including updated_at column. However, its parent (Questions table) does not update timestamp in updated_at column.</p>
  
  <p>How to solve this problem? Thank you.</p>