Mongodb: enron aggregation

db.messages.aggregate([
  { $unwind: "$headers.To"
  },
	{
		$project: {
			"_id": 1,
			"headers.From": 1,
			"headers.To": 1
		}
	},
	{
		$group: {
			_id: { id: "$_id", from: "$headers.From", to: "$headers.To" },
			count: { $sum: 1 }
		}
	},
	{
		$group: {
			_id: { from: "$_id.from", to: "$_id.to" },
			count: { $sum: 1 }
		}
	},
	{
		$sort: {
			count: -1
		}
	},
	{
		$limit: 5
	}
], {
  allowDiskUse:true,
  cursor:{}
 })