Fixing No Default Alternative Specified error in rails

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

No default alternative specified; choosing `x` as default

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

This is an A/B testing gem, and the error is 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 experiment 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