Cgridview Custom Filed

hi.
how can i add custom columns to CGridView, i want to get data from model by ActiveDataProvider and add one or more Columns to the dataProvider Array.
i do somthing like that :

'dataProvider'=>$dataProvider,
'columns'=>array(
      'name'=>'test',
      'type'=>'raw',
      'value'=>'some value'
)

but facing error that test (in this case) is not in my table(Model)