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db.collection.aggregate([{"$match":{$expr:{$let:{vars:{userIdInput:"6706c5576692b95bb39dabae"},"in":{"$or":[{$eq:["$$userIdInput","$from"]},{$in:["$$userIdInput","$to"]}]}}}}},{"$unwind":"$to"},{"$group":{"_id":null,"froms":{"$addToSet":"$from"},"tos":{"$addToSet":"$to"}}},{"$project":{allIds:{"$setUnion":["$froms","$tos"]}}}])