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db.users.aggregate([{"$lookup":{"from":"messages","let":{"username":"$username"},"pipeline":[{"$match":{"$expr":{"$eq":["$$username","$author"]}}},{"$group":{"_id":null,"highest_views":{"$max":"$views"}}}],"as":"highest_views"}},{"$set":{"highest_views":{"$arrayElemAt":["$highest_views.highest_views",0]}}}])