rails unit testing removes foreign key constraints in test database?

I've noticed that when testing is finished that at some point Rails has removed the foreign key constraints from my test database. Does anyone know where this foreign key constraint deletion occurs in the source code. I'd like to turn it off.

Thanks, Peter

After much hunting, I found the issue. I needed to add the following to my environment.rb file

  config.active_record.schema_format = :sql

Peter