code: public ForeignKey<Entity<T>> getOrCreatePrimaryKeyForeignKey()
     {
        Node node = childNode.getOrCreate("primary-key-foreign-key");
        ForeignKey<Entity<T>> primaryKeyForeignKey = new ForeignKeyImpl<Entity<T>>(this, "primary-key-foreign-key", childNode, node);
        return primaryKeyForeignKey;
     }
docs: If not already created, a new <code>primary-key-foreign-key</code> element with the given value will be created.
  Otherwise, the existing <code>primary-key-foreign-key</code> element will be returned.
  @return  a new or existing instance of <code>ForeignKey<Entity<T>></code>