code: public OQueryDataProvider<K> setContextVariable(String varName, IModel<?> value)
      {
      	model.setContextVariable(varName, value);
          super.detach();
          return this;
      }
docs: Set value for context variable
  @param varName name of the variable to set
  @param value {@link IModel} for the variable value
  @return this {@link OQueryDataProvider}