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db.collection.aggregate([{"$project":{"userWithRole":{"$map":{"input":"$userData","in":{"$let":{"vars":{"m":{"$arrayElemAt":[{"$filter":{"input":"$member","cond":{"$eq":["$$mb._id","$$this._id"]},"as":"mb"}},0]}},"in":{"$mergeObjects":["$$this",{"role":"$$m.role"}]}}}}}}}])