code: public UniqueConstraint<SecondaryTable<T>> getOrCreateUniqueConstraint()
     {
        List<Node> nodeList = childNode.get("unique-constraint");
        if (nodeList != null &&  nodeList.size() > 0)
        {
           return new UniqueConstraintImpl<SecondaryTable<T>>(this, "unique-constraint", childNode, nodeList.get(0));
        }
        return createUniqueConstraint();
     }
docs: If not already created, a new <code>unique-constraint</code> element will be created and returned.
  Otherwise, the first existing <code>unique-constraint</code> element will be returned.
  @return the instance defined for the element <code>unique-constraint</code>