Mongodb aggregation framework query (pairs of people that tend to communicate a lot)

people_most_communicate.js

use enron;

db.messages.aggregate([
	{ $unwind: '$headers.To' },
	{ $project: { _id: 0, 'headers.To': 1, 'headers.From': 1 } },
	{ $group: { _id: { to: '$headers.To', from: '$headers.From' }, count: { $sum : 1 } } }, 
	{ $sort: { count: -1 } },
	{ $limit: 5 }
]);