How to UPDATE DB values using active record?

[eluser]pernavoi[/eluser]
What is active record syntax for following query? amount1 and amount2 are INT values and I want to add values to them from data array

Code:

$this->db->query("UPDATE mytable
SET amount1 = amount1 + '{$data['amount1']}', amount2 =  amount2 + '{$data['amount2']}'
WHERE id = '{$data['id']}'");