MongoDB: Aggregate Array Values

mongo_aggregate.js

// ref: http://stackoverflow.com/questions/12654451/twitter-data-finding-the-most-mentioned-user-in-mongodb
// ref: http://stackoverflow.com/questions/12665866/mongodb-aggregating-array-items/12666156#comment17092191_12666156

var count = 0;
db.COLLECTION.find({key: "value"},{key_to_count:1}).forEach(
    function (doc) {
        count += doc.key_to_count.length;
    }
)
print("key_to_count: " + count);