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db.collection.aggregate([{$match:{"in_reply_to_user_name":{"$exists":true}}},{"$lookup":{"from":"collection","localField":"user_name","foreignField":"in_reply_to_user_name","as":"replies"}},{"$project":{"user_name":1,"replyCount":{"$size":"$replies"}}},{"$group":{"_id":"$user_name","replyCount":{"$first":"$replyCount"}}}])