code: @Override
      @SuppressWarnings("unchecked")
      public Q addJoinFlag(String flag, JoinFlag.Position position) {
          queryMixin.addJoinFlag(new JoinFlag(flag, position));
          return (Q) this;
      }
docs: Add the given String literal as a join flag to the last added join
  
  @param flag join flag
  @param position position
  @return the current object