If you get an error after you make a change, be sure to do the following: 1. Reset the Rails server. Use Control-C to stop the Rails server and then enter “rails server” to restart it. 2. Remove the contents of the tmp directory in your project. 3. Refresh your browser.
In other words, make sure that your problem isn’t due to anything cached that is outdated. If these steps don’t address your errors, then you’ll know that the error is from something else.
If you changed anything in the Gemfile, you need to also run “bundle install” (and maybe even “bundle update”).