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db.collection.aggregate([{"$project":{oldDocumentArr:{"$objectToArray":"$oldDocument"},newDocumentArr:{"$objectToArray":"$newDocument"},updatedFields:1}},{$project:{oldDocumentFilter:{$map:{input:{$filter:{input:"$oldDocumentArr",as:"item",cond:{$in:["$$item.k","$updatedFields"]}}},"as":"obj","in":{"k":"$$obj.k","v":{$toString:"$$obj.v"}}}},newDocumentFilter:{$map:{input:{$filter:{input:"$newDocumentArr",as:"item",cond:{$in:["$$item.k","$updatedFields"]}}},"as":"obj","in":{"k":"$$obj.k","v":{$toString:"$$obj.v"}}}}}},{$project:{oldDocument:{"$arrayToObject":"$oldDocumentFilter"},newDocument:{"$arrayToObject":"$newDocumentFilter"},_id:0}}])