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db.collection.aggregate([{"$lookup":{"from":"collection","localField":"replies","foreignField":"_id","as":"repliesPopulated"}},{"$addFields":{"repliesPopulated":{"$concatArrays":["$$ROOT.repliesPopulated",[{}]]}}},{"$unwind":{path:"$repliesPopulated",preserveNullAndEmptyArrays:true}},{"$replaceRoot":{"newRoot":{"$cond":{"if":{"$gt":["$repliesPopulated",{}]},"then":"$repliesPopulated","else":"$$ROOT"}}}},{"$unset":"repliesPopulated"}])