title: How can i update many to many relations in grails?
body: <p>I am doing a website with grails , I have used scaffoling plugin to make a CMS.
  I have a domain class activity that has 4 many-to-many relations and this class is the owner of all the relations. So when I edit a activity from my CMS, I have this:</p>
  
  <pre><code>http-bio-8080-exec-2] ERROR errors.GrailsExceptionResolver  - MySQLIntegrityConstraintViolationException occurred when processing request: [POST] /KurbanaProject/actividad/update/1
  Duplicate entry '1-2' for key 'PRIMARY'. Stacktrace follows:
  Message: Duplicate entry '1-2' for key 'PRIMARY
  </code></pre>
  
  <p>How can I update many to many relations?</p>
  
  <p>thanks</p>