Subqueries in activerecord

Rails now does this by default

Message.where(user_id: Profile.select("user_id").where(gender: 'm'))

will produce the following SQL

SELECT "messages".* FROM "messages" WHERE "messages"."user_id" IN (SELECT user_id FROM "profiles" WHERE "profiles"."gender" = 'm')

(the version number that “now” refers to is most likely 3.2)