title: How to find inverse of Eloquent query
body: <p>I've got a query to find enrolled students in a particular activity. On the User model:</p>
  
  <pre><code>public function enrolledStudents($activity)
  { 
      $students = $activity-&gt;students()
          -&gt;wherePivot('user_id', $this-&gt;id)
          -&gt;get();
      return $students;
  }
  </code></pre>
  
  <p>Where the Activity model's <code>students</code> method is this:</p>
  
  <pre><code>public function students()
  {
      return $this-&gt;belongsToMany('Student', 'activity_student', 'activity_id', 'student_id')
          -&gt;withPivot('user_id')
          -&gt;withTimestamps();
  }
  </code></pre>
  
  <p>I want another method to find the students who <em>aren't</em> enrolled in this activity - how could I go about this?</p>
  
  <p>I.e. $user->students()->notEnrolled($activity)</p>