mongodb/docs — pull request #292 by mikoskinen (2012-10-08 05:26:50+00:00)
Title: Change on how to do $group on compound keys

The currently documented example for creating a $group with compound keys doesn't work with 2.2. The change is based on this discussion: https://groups.google.com/forum/?fromgroups=#!topic/mongodb-user/1cYch580h0w