consistency_fail

https://github.com/trptcolin/consistency_fail

With more than one application server, validates_uniqueness_of becomes a lie. Two app servers -> two requests -> two near-simultaneous uniqueness checks -> two processes that commit to the database independently, violating this faux constraint. You'll need a database-level constraint for cases like these. consistency_fail will find your missing unique indexes, so you can add them and stop ignoring the C in ACID.