I am trying the creating custom grid columns, and found maybe man should append two line like this:
in controller loadModel() funciton.
because in function renderDataCellContent, there is one line like this:
$this->callbackUrl['pk'] = $data->primaryKey;
that means the ajax call with a parameter named pk, which should be checked and used in loadModel() function. otherwise there is 404 error, when man try to test the code in the book.