Mongoose / MongoDB: count elements in array

Aha, I've found the solution. MongoDB's aggregate framework allows us to execute a series of tasks on a collection. Of particular note is $unwind, which breaks an array in a document into unique documents, so they can be groups / counted en masse.

MongooseJS exposes this very accessibly on a model. Using the example above, this looks as follows:

Thing.aggregate([    { $match: { /* Query can go here, if you want to filter results. */ } }   , { $project: { tokens: 1 } } /* select the tokens field as something we want to "send" to the next command in the chain */  , { $unwind: '$tokens' } /* this converts arrays into unique documents for counting */  , { $group: { /* execute 'grouping' */          _id: { token: '$tokens' } /* using the 'token' value as the _id */        , count: { $sum: 1 } /* create a sum value */      }    }], function(err, topTopics) {  console.log(topTopics);  // [ foo: 4, bar: 2 baz: 2 ]});

It is noticeably faster than MapReduce in preliminary tests across ~200,000 records, and thus likely scales better, but this is only after a cursory glance. YMMV.