Existing Rails errors API is limiting for complex requirements, and AdequateErrors provides better alternative for error query, optional attribute prefix and object-oriented goodness.
Optimized operations on collection of Money objects
There are so many presenter/decorator gems aiming to achieve the same goal: bridging between model and view. The differences are mostly about implicit vs explicit interface design. This article lists many of the techniques used, so you will know what the gem owners have done in order to let you type less.