How to save parent and child in one shot (JPA & Hibernate)

Question

I start showing you my scenario.

This is my parent object:

@Entity
@Table(name="cart")
public class Cart implements Serializable{  

    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Id
    @Column(name="id")
    private Integer id; 

    @OneToMany(mappedBy="cart", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    private List<CartItem> cartItems; 

    ...
}

This is my child object:

@Entity
@Table(name="cart_item")
public class CartItem implements Serializable{  

    @GeneratedValue(strategy=GenerationType.IDENTITY)   
    @Id
    @Column(name="id")
    private Integer id;     

    @ManyToOne
    @JoinColumn(name="cart_id", nullable=false)
    private Cart cart;

    ...
}

As you can see looking at the database, in the table cart_item (child object) the field cart_id has a foreign key to the field id of the table cart (parent object).

This is how I save the object:

1) there's a restController that reads a JSON object:

@RestController
@RequestMapping(value = "rest/cart")
public class CartRestController {

    @Autowired
    private CartService cartService;    

    @RequestMapping(method = RequestMethod.POST)
    @ResponseStatus(value = HttpStatus.CREATED)
    public void create(@RequestBody CartDto cartDto) {
        cartService.create(cartDto);
    }
}

2) This is the CartService, that's just an Interface:

public interface CartService {  
    void create(CartDto cartDto); 
}

This is the implementation of CartService:

import org.springframework.transaction.annotation.Transactional;

    @Service
    @Transactional
    public class CartServiceImpl implements CartService {   
        @Autowired
        private CartDao cartDao;

        @Override
        public void create(CartDto cartDto) {
            cartDao.create(cartDto);
        }
    }

CartDao is just another interface, I show you only its implementation:

@Repository
public class CartDaoImpl implements CartDao {

    @Autowired 
    private SessionFactory sessionFactory;

    // in this method I save the parent and its children
    @Override
    public void create(CartDto cartDto) {       

        Cart cart = new Cart(); 

        List<CartItem> cartItems = new ArrayList<>();                   

        cartDto.getCartItems().stream().forEach(cartItemDto ->{     
            //here I fill the CartItem objects;     
            CartItem cartItem = new CartItem();         
            ... 
            cartItem.setCart(cart);
            cartItems.add(cartItem);                
        });
        cart.setCartItems(cartItems);

        sessionFactory.getCurrentSession().save(cart);                  
    }
}

When I try to save a new cart and its cart_items I get this error:

SEVERE: Servlet.service() for servlet [dispatcher] in context with path [/webstore] threw 
exception [Request processing failed; nested exception is 
org.springframework.orm.hibernate5.HibernateOptimisticLockingFailureException: Object of 
class     
[com.depasmatte.webstore.domain.CartItem] with identifier [7]: optimistic locking failed; 
nested exception is org.hibernate.StaleObjectStateException: Row was updated or deleted by 
another transaction (or unsaved-value mapping was incorrect) : 
[com.depasmatte.webstore.domain.CartItem#7]] with root cause
org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction
 (or unsaved-value mapping was incorrect) : [com.depasmatte.webstore.domain.CartItem#7]

I suppose the error depends on the fact that when Hibernate try to save the a cart_item, the id of the cart doesn't exist yet!

What's the correct way to save a parent object and its childer in on shot? Thank you

Answer 1

Here's the list of rules you should follow, in order to be able to store a parent entity along with its children in a one shot:

• cascade type PERSIST should be enabled (CascadeType.ALL is also fine)
• a bidirectional relationship should be set correctly on both sides. E.g. parent contains all children in its collection field and each child has a reference to its parent.
• data manipulation is performed in the scope of a transaction. NO AUTOCOMMIT MODE IS ALLOWED.
• only parent entity should be saved manually (children will be saved automatically because of the cascade mode)

Mapping issues:

• remove @Column(name="id") from both entities
• make setter for cartItems private. Since Hibernate is using its own implementation of the List, and you should never change it directly via setter
• initialize you list private List<CartItem> cartItems = new ArrayList<>();
• use @ManyToOne(optional = false) instead of nullable = false inside the @JoinColumn
• prefer fetch = FetchType.LAZY for collections

• it's better to use helper method for setting relationships. E.g. class Cart should have a method:

  public void addCartItem(CartItem item){
      cartItems.add(item);
      item.setCart(this);
  }
  

Design issues:

• it's not good to pass DTOs to the DAO layer. It's better to do the conversion between DTOs and entities even above the service layer.
• it's much better to avoid such boilerplate like method save with Spring Data JPA repositories