[{"order_id":1,"user_id":1001,"refund_id":123},{"order_id":2,"user_id":1002},{"order_id":3,"user_id":1001},{"order_id":4,"user_id":1001},{"order_id":5,"user_id":1003,"refund_id":111}]

db.collection.aggregate([{"$match":{"refund_id":123}},{"$lookup":{"from":"collection","localField":"user_id","foreignField":"user_id","as":"orders"}}])