Stubbing Rails.cache.fetch in Minitest

If you've got some code that's calling Rails.cache.fetch and you want to test that it's working properly, you can use the following method helper to stub the cache:

def stub_rails_cache(&block)
  @cache = {}

  cache_strategy = ->(key, &block) do
    @cache[key] ||= block.call
  end
    
  Rails.cache.stub :fetch, cache_strategy, &block
end

Which you can use in your tests:

stub_rails_cache do
  # In here, we're using an in-memory, naiive cache.
end

Note that there is another approach, but it doesn't work because of race conditions:

test "caches" do
  # Use in-memory cache
  Rails.cache = ActiveSupport::Cache::MemoryStore.new

  # Caching tests here, but if you use this approach in multiple
  # tests then race conditions could make this fail.
ensure
  # Reset cache
  store = Rails.application.config.cache_store
  Rails.cache = ActiveSupport::Cache.lookup_store(store)
end