InnoDB's foreign key support predates the existence of Rails by several years. However, InnoDB wasn't the default storage engine for MySQL at the time, so that may be a factor.
I wonder how different the state of database application development would be today if all those cheap whitelabel webhosts powered by cPanel or Plesk (where most of us got started, I imagine) opted for PostgreSQL instead of MySQL - which would have influenced the major MySQL adopters like phpNuke, phpBB, WordPress, etc.
