An around filter to temporarily apply changes
A client of mine has an admin tool where people can edit stuff, but modifications need approval. They are represented in the application rather than applied right away as you normally do.
Changes are basically stored in the form of method names, ID of receiver, and serialized arguments, there are a handful of them. Applying changes is dynamic invocation, you get the idea.
My client wanted an approval interface where he could select a handful of changes to the same model, submit, and get a split view with the current website on the left, and the resulting website on the right.
The tricky part is that views may access the database, for example using named scopes (from a MVC viewpoint that's fine, the view is clean it happens that a named scope triggers a query). And the 3-line solution was to write an around filter that started a transaction, yielded to the action, and rolled the transaction back.