When using memcached, if you store Model objects in the cache, when you go to load to object back out it doesn't know about your Model classes and throws an 'Undefined Class/Module' error. The old solution for this was to add the 'model' method to the top of your controller and list the models that were needed. This is now deprecated.
before_filter :preload_models def preload_models() Model1 Model2 ... ... ... Model9 end
What it do?
This is more of a hack than a solution, but it works. Add the above code to your application controller and list all the models you are storing in memcached as well as any associations that are also being stored. Referencing the Model class name must trigger a load that loading from memcached does not.