rails/rails — pull request #18238 by aaron-coding (2014-12-29 15:58:25+00:00)
Title: Include error message for first! to match last!

Check out this SO question: http://stackoverflow.com/questions/27627812/first-on-ar-collectionproxy-raises-undefined-method-for-nil-after-updati/27691893#27691893

Looks like the first! method should be re-implemented to give errors, as would be expected with a bang method.