$this->validation->field_value problem

[eluser]Computerzworld[/eluser]
I am having a combobox which is created in controller dynamically on change event of other combo and passed to view using ajax. But when I select its value and if I have not entered other field's value and if I submit the form the combo gets deselected again. I have written $this->validation->combovalue = $data->combovalue for the selection purpose. But it doesn't works in my controller. It works perfectly in view. How can I solve it? please help me. Thanks in advance.