title: Makumba list sorting by a subset
body: <p>Let's say we have the following Student.mdd:</p>
  
  <pre><code>person=not null ptr general.Person
  interests=set general.ActivityField
  education=set
    education-&gt;university=not null ptr general.University
    education-&gt;graduation=date
  </code></pre>
  
  <p>I would like to list all students who are interested in painting (where="interests.name='painting'") and sort the list by the latest graduation date.  </p>
  
  <p>If I simply use orderBy="student.education.graduation desc", only the first created education will be used for sorting. For example:</p>
  
  <ul>
  <li>Student1 went to University A and graduated in June 2009</li>
  <li>Student2 went to University B and graduated in June 2010</li>
  <li>Student3 went to University C and graduated in May 2010 and then went to University D and graduated in May 2011</li>
  </ul>
  
  <p>So I expect to get them sorted like this:</p>
  
  <ol>
  <li>Student3 (May 2011)</li>
  <li>Student2 (June 2010)</li>
  <li>Student1 (June 2009)</li>
  </ol>
  
  <p>But I get this:</p>
  
  <ol>
  <li>Student2 (June 2010)</li>
  <li>Student3 (May 2010)</li>
  <li>Student1 (June 2009)</li>
  </ol>
  
  <p>Is there a way to force makumba to go through all "education" items for each student, sort them by "graduation" and use the latest date to sort the student?</p>