Show values from more than just the first column when establishing many to many relationships

When establishing many to many relationships between custom tables and e.g. Features, it would be nice to have an option to select columns from custom table to be displayed in the combo box, because showing just values from the first column is often inconvenient, e.g. if the fist column is unique, then it is probably not descriptive enough; or if the fist column is not unique, the user is not able to establish relation base on this value only.