public Relation Relation(
     Project project,
     String leftName,
     Domain left,
     Cardinality leftCardinality,
     String rightName,
     Domain right,
     Cardinality rightCardinality,
     Expression join) {
   RelationPK pk = new RelationPK(project.getId(), getNewID());
   Relation relation =
       new Relation(
           pk,
           left.getId(),
           leftCardinality,
           right.getId(),
           rightCardinality,
           leftName,
           rightName,
           join);
   RelationServiceBaseImpl.getInstance().store(ctx, relation);
   return relation;
 }