How do you run a single test/spec file in RSpec?
The raw invocation:
rake spec SPEC=spec/controllers/sessions_controller_spec.rb \ SPEC_OPTS="-e \"should log in with cookie\""
Now figure out how to embed this into your editor.
Or you can skip rake and use the 'rspec' command:
bundle exec rspec path/to/spec/file.rb
In your case I think as long as your ./spec/db_spec.rb file includes the appropriate helpers, it should work fine.
If you're using an older version of rspec it is:
bundle exec spec path/to/spec/file.rb