My application uses :mem_cache_store for fragment caching, but not for caches_action or caches_page.
Running my app against svn revision 6832, my tests run fine. If I update to revision 6833 or any later revision at current time of writing, I get the following exception when I attempt to run any test:
/app/vendor/rails/activerecord/lib/../../activesupport/lib/active_support/dependencies.rb:477:in `const_missing': uninitialized constant ActionController::Caching::Fragments::MemCacheStore::MemCache (NameError)
from /app/vendor/rails/actionpack/lib/action_controller/caching.rb:484:in `initialize'
from /app/vendor/rails/actionpack/lib/action_controller/caching.rb:338:in `new'
from /app/vendor/rails/actionpack/lib/action_controller/caching.rb:338:in `fragment_cache_store='
... etc ...
The line in caching.rb in question is:
@data = MemCache.new(*addresses)
so it would appear to be a simple problem of the memcache libraries not being loaded.
The problem does not appear to affect the app running in development or production mode.
Changeset 6833 introduced demand-loading in ActiveRecord but did not touch ActionController.