Database
db={"order":[{"itemId":"5a934e000102030405000000","price":1,"quantity":2},{"_id":2,"itemId":"5a934e000102030405000001","price":2,"quantity":1},{"_id":3,"itemId":"5a934e000102030405000002","price":1,"quantity":2}],"item":[{"_id":ObjectId("5a934e000102030405000000"),"name":"apple"},{"_id":ObjectId("5a934e000102030405000001"),"name":"orange"},{"_id":ObjectId("5a934e000102030405000002"),"name":"banana"}]}
Query
db.order.update({},[{$set:{itemId:{$toObjectId:"$itemId"}}}])