[philwelch]

Rails not only has to consult the database to validate uniqueness, but it still doesn’t know for sure even then whether the row will remain valid. So why not just insert the row and see if you get away with it?

Simply removing the uniqueness validation and relying exclusively on database indices would be strictly better. ActiveRecord dynamically infers from the DB schema which columns your tables have; it could also infer foreign key relationships and uniqueness guarantees and use them to generate pre-insertion “best guesses” about validity if the user wants to run those without necessarily inserting. But Rails is supposed to be an opinionated framework, and “just try to insert and we’ll let you know how that worked out for you, since we can’t make any guarantees otherwise” is a perfectly valid opinion.