code: @SuppressWarnings("unchecked")
  	public OQueryModel<K> setContextVariable(String varName, IModel<?> value)
      {
          variables.put(varName, (IModel<Object>)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 OQueryModel}