addItem.java

@Override
public void addItem(String product) {
	log.info("Create item for product "+product);
	Item i = new Item();
	i.setProduct(product);
	i.setOrder(o);
	o.getItems().add(i);
	this.em.persist(i);
}

createItems.log

14:30:17,539 INFO  [ShoppingCart] (default task-1) Create item for product myFirstProduct
14:30:17,540 DEBUG [org.hibernate.SQL] (default task-1) 
    select
        nextval ('hibernate_sequence')
        
14:30:17,548 INFO  [ShoppingCart] (default task-1) Create item for product mySecondProduct
14:30:17,548 DEBUG [org.hibernate.SQL] (default task-1) 
    select
        nextval ('hibernate_sequence')

createOrder.log

14:30:17,454 INFO  [ShoppingCart] (default task-1) Create order
14:30:17,464 DEBUG [org.hibernate.SQL] (default task-1) 
    select
        nextval ('hibernate_sequence')

FillShoppingCart.java

// 1. create the Order
cart.start();

// 2. add one Item to the Order
cart.addItem("myFirstProduct");
// 3. add additional Item to the Order
cart.addItem("mySecondProduct");

// 4. write Order with its Items to the database
cart.persist();

Item.java

@Entity
public class Item {

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	@Column(name = "id", updatable = false, nullable = false)
	private Long id;
	
	@Column
	private String product;

	@ManyToOne
	@JoinColumn(name = "fk_order")
	private Order order;
	
	...
}

itemsAfterJoin.log

14:30:17,748 DEBUG [org.hibernate.SQL] (default task-1) 
    select
        item0_.id as id1_0_,
        item0_.fk_order as fk_order3_0_,
        item0_.product as product2_0_ 
    from
        Item item0_

14:30:17,756 DEBUG [org.hibernate.SQL] (default task-1) 
    select
        order0_.id as id1_1_0_ 
    from
        my_order order0_ 
    where
        order0_.id=?

14:30:17,765 INFO  [stdout] (default task-1) After joinTransaction: 2 items.

itemsBeforeJoin.log

14:30:17,681 DEBUG [org.hibernate.SQL] (default task-1) 
    select
        item0_.id as id1_0_,
        item0_.fk_order as fk_order3_0_,
        item0_.product as product2_0_ 
    from
        Item item0_
        
14:30:17,694 INFO  [stdout] (default task-1) Before joinTransaction: 0 items.

joinTransaction.log

14:30:17,694 INFO  [ShoppingCart] (default task-1) Join transaction and write changes to the database
14:30:17,728 DEBUG [org.hibernate.SQL] (default task-1) 
    insert 
    into
        my_order
        (id) 
    values
        (?)

14:30:17,739 DEBUG [org.hibernate.SQL] (default task-1) 
    insert 
    into
        Item
        (fk_order, product, id) 
    values
        (?, ?, ?)

14:30:17,743 DEBUG [org.hibernate.SQL] (default task-1) 
    insert 
    into
        Item
        (fk_order, product, id) 
    values
        (?, ?, ?)

Order.java

@Entity(name="my_order")
public class Order {
	   
	@Id
	@GeneratedValue(strategy = GenerationType.SEQUENCE)
	@Column(name = "id", updatable = false, nullable = false)
	private Long id;

	@OneToMany(mappedBy = "order")
	private List<Item> items = new ArrayList<Item>();
	
	...
}

persist.java

public void persist() {
	log.info("Join transaction and write changes to the database");
	this.em.joinTransaction();
}

PersistenceContext.java

@PersistenceContext(synchronization = SynchronizationType.UNSYNCHRONIZED, type = PersistenceContextType.EXTENDED)
EntityManager em;

ShoppingCart.java

@Stateful
public class ShoppingCart implements ShoppingCartRemote {

	Logger log = Logger.getLogger(this.getClass().getSimpleName());
	
	@PersistenceContext(synchronization = SynchronizationType.UNSYNCHRONIZED, type = PersistenceContextType.EXTENDED)
	EntityManager em;
	Order o;
	
	@Override
	public void start() {
		log.info("Create order");
		this.o = new Order();
		this.em.persist(o);
	}
	
	@Override
	public void addItem(String product) {
		log.info("Create item for product "+product);
		Item i = new Item();
		i.setProduct(product);
		i.setOrder(o);
		o.getItems().add(i);
		this.em.persist(i);
	}
	
	@Override
	public void persist() {
		log.info("Join transaction and write changes to the database");
		this.em.joinTransaction();
	}
}

start.java

public void start() {
	log.info("Create order");
	this.o = new Order();
	this.em.persist(o);
}