Originally introduced in this thread: https://forum.red-gate.com/discussion/comment/149220#Comment_149220

This is a common problem for us when using Data Compare. Parent table has unique constraint on a char field alongside an identity field which also serves as a one-to-many foreign key in the child table. The child table uses an identity field to distinguish each row but there's no guarantee the id's will be the same across databases so only the parent can be matched but the child table only has the parent_id identity fk which is not a viable comparison key.

My proposed solution would be to allow user to define a computed comparison key which would output a key to use for matching for each row in the table.