public Merge::keys(array $fields, array $values = array())
Sets the key fields to be used as conditions for this query.
This method should only be called once. It may be called either with a single associative array or two indexed arrays. If called with an associative array, the keys are taken to be the fields and the values are taken to be the corresponding values to set. If called with two arrays, the first array is taken as the fields and the second array is taken as the corresponding values