Yeah, as others have noted, the default for Rails is definitely not Mongo. SQLite is the default and thus ends up getting used a lot in development. Production

Yeah, as others have noted, the default for Rails is definitely not Mongo. SQLite is the default and thus ends up getting used a lot in development. Production is a relatively even split between Postgres and MySQL, although I think it tilts slightly towards Postgres in part because that's the default on Heroku. (I have no data to back this up, just my sense of things.)