title: Cakephp 3 NOT IN query
body: <p>I think this is a common pattern, but I can't find the elegant CakePHP way of doing it. The idea is to remove the values from a list which have already been chosen. To use an example from the Cake book:</p>
  
  <pre><code>table Students (id int, name varchar)
  table Courses (id int, name varchar)
  table CoursesMemberships (id int, student_id int, course_id int, days_attended int, grade varchar)
  </code></pre>
  
  <p>All I want to do is create a query which returns the courses that a given student has not yet signed up for, most probably to populate a select dropdown.</p>
  
  <p>If I weren't using Cake, I'd do something like</p>
  
  <pre><code>select * from Courses where id not in 
      (select course_id from CoursesMemberships where student_id = $student_id)
  </code></pre>
  
  <p>Or maybe an equivalent NOT EXISTS clause, or outer join trickery, but you get the idea.</p>
  
  <p>I'm stumped how to do this elegantly in Cake. It seems to me that this would be a common need, but I've researched for awhile, as well as tried some query ideas, to no avail.</p>