Call back before insert not work in Grocery CRUD

Question

My Controller, inserts a row in invHdrData Table, i want the Max Doc Number in that Table to insert a Doc Number automatically The callback before insert not works. So Where is the problem? I would greatly appreciate it if you kindly give me some answers :)

My Controller :

        public function sanads()
{
        $crud = new grocery_CRUD();
        $crud->set_theme('datatables');
        $crud->set_table('invhdrdata');
        $crud->set_subject('BLAH BLAH BLAH');
        $crud->columns('FiscalYear','StoreNo','DocType','CreateDate','UpdateDate');
        $crud->fields('FiscalYear','StoreNo','DocType','DocNo','CreateDate','UpdateDate');
        $crud->callback_before_insert(array($this,'maxDocNoCon'));
        $crud->field_type('DocNo','invisible');
        $output = $crud->render();
        $this->_example_output($output);
}

Another Function in Controller:

    public function maxDocNoCon($post_array) {
    $this->load->model('inv_model');
    $post_array['DocNo'] = $this->inv_model->maxDocNoMod($post_array['FiscalYear'],$post_array['StoreNo'],$post_array['DocType']);
    $test['DocNo'] = (int)explode(",", $post_array['DocNo'])+1;
    return $test;
}

My Model:

    function maxDocNoMod($FiscalYear,$StoreNo,$DocType){
    $this->db->select_max('DocNo');
    $this->db->where('FiscalYear', $FiscalYear);
    $this->db->where('StoreNo', $StoreNo);
    $this->db->where('DocType', $DocType);
    $data = $this->db->get('invhdrdata');
    if ($data->num_rows() != 1) {
        // there should only be one row - anything else is an error
        return false;
    }
    return $data->row()->DocNo;
}

Answer 1

Problem Solved. I changed the controller to the following:

    public function maxDocNoCon($post_array) {
    $this->load->model('inv_model');
    $post_array['DocNo'] = $this->inv_model->maxDocNoMod($post_array['FiscalYear'],$post_array['StoreNo'],$post_array['DocType'])+1;
    if ($post_array['DocNo'] == 1){
        $post_array['DocNo'] = 10001;
    }
    return $post_array;
}