Setting the flag allowDiskUsage to true seems to not be working for me, returning the following message "Exceeded memory limit for $group, but didn't allow external sort. Pass allowDiskUse:true to opt in".

According to the main mongo docs (http://docs.mongodb.org/manual/reference/method/db.collection.aggregate/), the flag name is "allowDiskUse" and not "allowDiskUsage" as used in the lib/mongodb/aggregation_cursor.js.