Need help on converting mysql query to codeigniter active record

Code:

SELECT date(`timestamp`)
     , COUNT(`odr_id`) AS num_sales
     , SUM(`amnt_paid`) AS daily_total
     , SUM(gov_fee) as e_pay
     , SUM(items_total) as sum_tot_cost
  FROM item_table
GROUP
    BY date(`timestamp`)
where ('user', $this->session->userdata('usernm'));

I get the username as table column and throws error saying "column not found"