ActiveRecord::Fixtures.create_fixtures and RI_ConstraintTrigger error

When you run ActiveRecord::Fixtures.create_fixtures with postgresql adapter, it requires the db user (in config/database.yml) to be superuser. So the user is created by 'createuser -S' command, ActiveRecord::Fixtures.create_fixtures will fail with permission denied error related to RI_ConstraintTrigger. There are some useful discussion over here: https://github.com/matthuhiggins/foreigner/issues/61