title: Set::extract Cakephp change format of the returned array
body: <p>I am new to cake so please be patient.
  I have this array </p>
  
  <pre><code>Array
  (
  [0] =&gt; Array
      (
          [CoursesLessons] =&gt; Array
              (
                  [id] =&gt; 108
                  [course_id] =&gt; 88
                  [lesson_id] =&gt; 6
                  [deleted] =&gt; 
              )
  
          [UserLessonsCompleted] =&gt; Array
              (
                  [0] =&gt; Array
                      (
                          [id] =&gt; 2
                          [courses_lessons_id] =&gt; 108
                          [user_id] =&gt; 75
                          [created] =&gt; 2015-01-27 14:04:31
                          [modified] =&gt; 0000-00-00 00:00:00
                          [completed_at] =&gt; 2015-02-03 14:04:45
                          [homework_downloaded_at] =&gt; 0000-00-00 00:00:00
                          [deleted] =&gt; 
                      )
  
              )
  
      )
  
   //and so on
  </code></pre>
  
  <p>I would like to exctract the <code>UserLessonsCompleted completed at</code>.
  What I am trying to do is: </p>
  
  <pre><code>$stringOfIdsCompleted =  Set::extract($coursesLessons, '{n}.UserLessonsCompleted.{n}.completed_at');
  </code></pre>
  
  <p>my problem is that they array that is returned is: </p>
  
  <pre><code>Array
  (
   [0] =&gt; Array
      (
          [0] =&gt; 2015-02-03 14:04:45
      )
  
  [1] =&gt; Array
      (
          [0] =&gt; 2015-02-06 15:08:01
      )
  //and so on
  </code></pre>
  
  <p>while I would like something as:</p>
  
  <pre><code> Array
   (
   [0] =&gt; Array
      (
          [completed_at] =&gt; 2015-02-03 14:04:45
      )
  
  [1] =&gt; Array
      (
          [completed_at] =&gt; 2015-02-06 15:08:01
      )
  </code></pre>
  
  <p>Any help on how to achieve this? thank you in advance for help</p>
  
  <p>here is how i retrieve my data:</p>
  
  <pre><code>$coursesLessons = $this-&gt;CoursesLessons-&gt;find('all', array(
          'conditions' =&gt; array(
              'CoursesLessons.course_id' =&gt; $courseId,
          ),
          'contain' =&gt; array(
              'UserLessonsCompleted' =&gt; array(
                  'conditions' =&gt; array(
                      'UserLessonsCompleted.user_id' =&gt; $userId,
                  ),
              ), 
          )
      ));
  </code></pre>