[{"application_detail":{},"curl_detail":{"Curl1":{"key1":"value1","key2":"value2"},"Curl2":{"key1":"value1","key2":"value2"},"Curl3":{"key1":"value1","key2":"value2"},"Curl4":{"key1":"value1","key2":"value2"}}}]

db.collection.aggregate([{$addFields:{curl_detail:{$arrayToObject:{$map:{input:{$objectToArray:"$curl_detail"},as:"details",in:{k:"$$details.k",v:{key1:"$$details.v.key1"}}}}}}},{$project:{_id:0,curl_detail:1}}])