Today I solved an interesting problem in Ruby on Rails.

When I upgraded an application to a new rails version the application worked but some tests failed with “No route matches “. I couldn’t figure out why. So I wanted to trace the failure down to a a specific rails commit.

I did this by using the following line in the Gemfile: gem ‘rails’, :git => “git://”, :ref => “8af2fd889”

I varied the reference to track down the offencing commit and each time ran: bundle update rails && rspec rspec spec/controllers/example_controller_spec.rb

Normally I use git bisect but wouldn’t know how to do this in case there is another repository causing the problems. I guess you have to point bundler to a local rails repo that you run git bisect in.

In the end the problem was that format parameter was not stringified in JSON requests due to rails commit:

To fix it I had to change: :format => format to ‘format’ => format