code: @Override
      public boolean add(D item) throws UnsupportedOperationException {
          readOnlyGuardCondition();
          instance.addRelation(entity, writeAttributeName, item);
          entity.save();
          return true;
      }
docs: Add item to collection.
  
  @param item item for addition (must be Entity or subclass).
  @return if item add then will return true in the other case false.
  @throws UnsupportedOperationException if object is read only.