[{obj1:{code:"123"},obj2:{code:"654"},obj3:{code:"abc"},obj4:{code:"xyz"},type:"obj1",},{obj1:{code:"123"},obj2:{code:"654"},obj3:{code:"abc"},obj4:{code:"xyz"},type:"obj2",},{obj1:{code:"123"},obj2:{code:"654"},obj3:{code:"abc"},obj4:{code:"xyz"},type:"obj4",}]

db.collection.aggregate({$addFields:{code:{$let:{vars:{match:{$first:{$filter:{input:{$objectToArray:"$$ROOT"},cond:{$eq:["$type","$$this.k"]}}}}},in:"$$match.v.code"}}}})