Rails is generally designed assuming that the application will get its own database. This is normal in environments where "database" is a lightweight concept, as in MySQL, PostgreSQL, or SQLite. After updating the database, it dumps the database schema to a file so when you install a new copy of the application you don't need to run dozens of migrations.
This works fine when there aren't many tables unrelated to your application, but in Oracle-land it's not unusual to put tons of applications all in the same massive database. Consider the following:
Every time I add a single column to the database via migrations, all those tables get introspected and transformed into 13.5K lines of ruby. A three minute tax is added to an operation which usually takes less than a second.
I wonder if there's some way to turn off this behavior without hacking up their Rake file.
Update: Turns out this only happens if ActiveRecord.schema_format = :ruby. Change it to :sql and it won't try to dump the database.