Expressions, Parent, Master-details

This is not so well known problem. Probably because it is weird to do it so. Anyway, it does not work.

I had master/detail relationship between two tables. In detail table I had expressions like
Parent("groups_users").Name
Which means I tried to get value from parent record using relation "groups_users". Unfortunately, when I inserted record into parent table, and then inserted record into detail table without posting data to server, I got exception that "Original value is not available". It is true, that original value is not available for new records, it is just unclear why current value is not taken.

I could not find workaround with expressions and gave up, retrieving data manually.