Column 'AssignedToId' does not exist.

[BananaHsu]

Hi, I'd like to update the data to someone else with his/her email id. Although the column displayed on the Web site is "AssignedTo", I got "AssignedToId" and "AssignedToStringID" by using item.properties.keys(). So I try to update the key "AssignedToId", but it pops up the error saying it doesn't exist. Is it correct to use properties.keys() to get the column list? or is there any other way I can get the correct keys for "AssginedTo".