title: JPA EclipseLink ManyToMany with Data
body: <p>I'm currently trying to implement a ManyToMany Relationship with Data in the JoinTable.
  I'm following <a href="http://boris.kirzner.info/blog/archives/2008/07/19/hibernate-annotations-the-many-to-many-association-with-composite-key/" rel="nofollow">this approach</a>  with the Eclipselink JPA Framework.
  But I'm getting the following exception:</p>
  
  <blockquote>
    <p>org.eclipse.persistence.exceptions.EntityManagerSetupException.predeployFailed(EntityManagerSetupException.java:210)
        ... 23 more
    Caused by: Exception [EclipseLink-7298] (Eclipse Persistence Services - 2.1.1.v20100817-r8050): org.eclipse.persistence.exceptions.ValidationException
    Exception Description: The mapping [group] from the embedded ID class [class de.kapieren.mbm.server.model.UserGroupPK] is an invalid mapping for this class. An embeddable class that is used with an embedded ID specification (attribute [pk] from the source [class de.kapieren.mbm.server.model.GroupMembership]) can only contain basic mappings. Either remove the non basic mapping or change the embedded ID specification on the source to be embedded</p>
  </blockquote>
  
  <p>Does anybody know whats the meaning of basic mappings in relationship to embeddedId?
  What could be wrong here?</p>