Has the actual philosophy changed though?

When I first got to Rails it seemed that databases were considered as a bothersome but unavoidable necessity; flat files with a funny accent, instead of an essential and powerful ally in the fight against entropy and error.

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.

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.

That looks mostly like you've added support for postgres "extras", not (inherent) "postgres goodness". Not that adding support for "extras" is a bad thing -- but in the context of "adding support for postgres sql" -- that seems somewhat orthogonal? (I'm not saying it is wrong for rails to approach postgres this way, just that the approach doesn't seem to change the way rails (afaik) approach databases: logic should be in rails, not in stored procedures and views (which is where it would make sense to place logic, if you design with "database first")).

Interesting link, either way.