The first thing we do in this method is that we find the record in CarTable by
using the carId in the record that is selected in the form. If the field is edited, the set
parameter is true; we can use that parameter to decide if the record we select from
CarTable should be selected for update or not.
If the set variable is true we then tell the server to lock the record so that only
this process can update the record by using the ttsbegin statement. You will
learn more about manipulating data later in the book, so I won't go into details
here, but the Mileage field in CarTable is set to be equal to the value that the
user entered in the RentalTable. The ttscommit will make sure that the record is
written to the database, and will also unlock the record so that it can be updated by
other processes.