The error message states that the record you are attempting to update has no 'id' field specified - you can have a 'candidate_id' field as well, if you want, but there must always be a field called 'id' (which must always be an auto-incrementing integer and the primary key for the table).
Assuming you use the XMLDB editor to write your DB install/upgrade code (which you always should do), this will be added for you.