Appalling Farrago

This article was originally posted on the thoughtbot blog here

I ran into a problem on a Rails app recently where tests were leaking state via caching. Typically you may see the following configuration in config/environments/test.rb:

Rails.application.configure do
  config.action_controller.perform_caching = false
end

However, this only disables controller caching. If you application has fragment caching, custom caching behavior, or you are using a gem with caching, then the above may not stop caching behavior between test runs. For me, this came up as a test that would fail, but only if a previous test that hit the same page was run immediately before it. And even then it only failed sometimes.

It turns out that the page being tested was using fragment caching. The cache key depended on the updated_at and id of the record being shown. If the previous test ran in less then a second, the following test would hit the same page and see the cached result.

Instead, you should have the following line in config/environments/test.rb:

Rails.application.configure do
  config.cache_store = :null_store
end

This will have caching on but any writes to the cache are immediately discarded.

There is a trade off here of time vs. coverage. Using :null_store saves us time at the expense of not testing reads from the cache. This means that we need to be careful of making sure that cache keys are truly unique.