Without going into details here, it was my understanding that your add_unique_constraint must be reflected in your database by adding a UNIQUE INDEX on these fields so the database engine will loudly complain if you add duplicate entries. I did not think DBIx::Class will by itself check these unique constraints and will rely on the much more optimized routines of the database engine.

Then again, I did not study the innards of DBIx::Class so I may be miles off the mark.