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db.collection.aggregate([{$set:{thoughts:{$map:{input:"$thoughts",as:"thought",in:{$mergeObjects:["$$thought",{replies:{$map:{input:"$$thought.replies",as:"reply",in:{$mergeObjects:["$$reply",{username:{"$cond":{"if":"$$reply.anonymous","then":"anonymous","else":"$$reply.username"}}}]}}}}]}}}}}])