[integraton]

In the past several years Postgres and NoSQL data stores have gained in popularity and become mainstream, and Rails now supports them well. The Rails ecosystem historically focused on MySQL, but that's not the case anymore. For example, Engine Yard now uses Postgres by default, and Heroku always has.

[jacques_chester]

I basically remember head-scratching about the worthiness of exotic features like foreign keys.

Some poking around in the current documentation seems to suggest that FKs have been absorbed into ActiveRecord.