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