Java – Data transfer object in dao design pattern

daodesign-patternsjava

I am bit confused about what data should a DTO contain.
For example let's assume that we have two tables: User, and Orders.
Orders table contains id_users, which is foreign key to user table.

Obviously I have two DAOs, MysqlUserDao and MysqlOrdersDao, with crud operations, and two transfer objects User, and Order, in which I store jdbc rowset.

If I want to get the list of users and for each user all his orders how should I do:

1) In my MysqlUserDao create a function: getUsersAndOrders(select users.,orders. from users join orders)
And my User DTO should have a OrderList property in where i put orders ?

2) In my MysqlUserDao i create a function getAllUsers(select * from users),
and foreach user I use MysqlOrdersDao function getOrder(id_user);

And some clarifications:

1) For each table in database I need to create a DAO object? or just for complex ones?
For example products and images, should be 2 dao or just one?

2) a DTO object should have only properties and setter getter, or it is possible to have other methods like convertEuroToUsd etc.

thanks

Best Answer

In your scenario #1 is the best option because #2 generates too much overhead.

1) In my MysqlUserDao create a function: getUsersAndOrders(select users.,orders. from users join orders) And my User DTO should have a OrderList property in where i put orders ?

Clarifications: 1: If your database has a good Design, then a DAO for each table is a good approach. There some cases where you can merge DAOs together (e.g: inheritance).

2: Yes. It should be a plain bean (or POJO if you want). I suggest creating another layer where you can define your workflow. I've seem people calling this extra layer as model, sometimes DataManager, sometimes just Manager.

For instance: When creating a order you should insert a record in Order table and also insert a record in the Notification table (because end users will be notified via email every time a order is created)

class OrderManager {
   private OrderDAO oDao;
   private NotificationDao nDao;

   public saveOrder(OrderDTO o) {
      Long orderId = oDao.save(o);
      NotificationDTO n = new NotificationDTO();
      n.setType(NotificationType.ORDER_CREATED);
      n.setEntityId(orderId);
      nDao.save(n);
   }
}

UPDATE: In most cases we can say that:

  • "Managers" may handle many DAOs;
  • DAOs should not contain other DAOs and are tied to a DTO;
  • DTOs can contain other DTOs

There is an important idea of LAZY or EAGER load when it comes to handling collections. But this is another subject :D

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