[/** 1*/{"_id":ObjectId("62622dd73905f04f59db2971"),"array1":[{"_id":"21","array2":[{"_id":"123","answeredBy":["success"]},{"_id":"124","answeredBy":[]}]}]},/** 2*/{"_id":ObjectId("626230e03905f04f59db29f5"),"array1":[{"_id":"22","array2":[{"_id":"223","answeredBy":[]},{"_id":"220","answeredBy":[]}]}]}]

db.collection.update({"array1._id":{"$exists":true}},[{"$set":{"array1":{"$map":{"input":"$array1","as":"elem","in":{"$mergeObjects":["$$elem",{"_id":{"$toInt":"$$elem._id"}}]}}}}}],{"multi":true})