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

Make sure that your method is Transactional. you can make method Transactional using @Transactional annotation on top of method signature.

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

For bidirectional relation act like below:

1. Set cascade to persist or All
2. Remove mappedBy attribute in @OnToMany if there is
3. Write @JoinCloumn at both sides (otherwise it creates Join Table) with the same name
4. Remove (nullable = false) in @JoinColumn (because Hibernate first inserts the parent record then inserts child records and after all, updates the foreign key in child records)

Here is the sample code:

public class Parent {

    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
    @JoinColumn(name = "fk_parent")
    private List<Child> children;

}

public class Child {

    @ManyToOne
    @JoinColumn(name = "fk_parent")
    private Parent parent;

}