I've been working on a Drupal module which aggregates and reports on some data from Mongo. In order to keep from having to re-do all the aggregations, the aggregated data itself is written off to a Mongo collection which is then retrieved and displayed when the reports are requested. To make the retrieval more efficient, I need an index on the table storing the aggregated data.
One of the systems I've lately inherited makes heavy use of Mongo for data storage, a data system I've not used previously. So, when the boss called tonight and said that his boss needed counts of an object in our system by state in the next 10 minutes my thinking went something like...
No problem, that's a simple SQL group-by....Oh, wait. This is mongo. Oh, crap! How do I do that?! It's a function; Map...Something.