Another fine open-source contribution from @aaw, mongoid_collection_snapshot: easy maintenance of collections of processed data in MongoDB with the Mongoid ODM.
github.com/aaw/mongoid_collection_snapshot
This turned out to be extremely useful in many map-reduce scenarios where we do lengthy computations that output data that is used as a complete and consistent set. For example, calculating average prices across ranges of items (the example in the documentation). Another good use can be for tags stored as arrays of strings – you want to know how many items are tagged with a given entry. We can now take my incremental tag count map-reduce and replace it with a much more trivial implementation that uses this gem.