If when you start up Rails locally you see the message;

  No default alternative specified; choosing `x` as default

Then this means you probably have the Vanity Gem installed in your project.

This is an A/B testing gem, and the error is basically saying that in one of your experiments you have not specified a default. When this happens the first of the alternatives is picked.

For example; if you had the following in an experiement file;

  ab_test 'My AB Test' do
    description 'Sample Test'
    alternatives 'First', 'Second', 'Third'
    metrics :my_metric
  end

I would expect you to get an error on startup of your Rails server saying;

  No default alternative specified; choosing First as default

In order to fix this we simply need to specify a default

  ab_test 'My AB Test' do
    description 'Sample Test'
    alternatives 'First', 'Second', 'Third'
    metrics :my_metric
    default 'Third'
  end

Why not read some more of our Ruby posts?