@ManyToOne(updatable=false) - how it should work?

问题

I want to have a read-only functionality in one of my entities. I know that in JPA 2.0 we don't have such functionality per se. I thought we can achieve it using updateable=false, insertable=false but I don't think I get how it works.

Assume that I have two entities: OrderedItem and Customer:

@Entity
public class OrderedItem {

    @Id
    @GeneratedValue
    private int id;

    private String name;

    @ManyToOne
    @JoinColumn(updatable = false)
    private Customer owner;

    // bunch of simple getters and setters
}

@Entity
public class Customer {

    @Id
    @GeneratedValue
    private int id;

    private String name;

    @OneToMany(mappedBy="owner")
    private Set<OrderedItem> orderedItems;

    // bunch of simple getters and setters
}

Now consider the following code:

Customer john = new Customer();
john.setName("John");

OrderedItem milk = new OrderedItem();
milk.setName("Milk");
milk.setOwner(john);

Set<OrderedItem> items = new HashSet<OrderedItem>();
items.add(milk);        
john.setOrderedItems(items);

// This starts the EM transaction
startTx();

em.persist(john);
em.persist(milk);

stopTx();

startTx();

OrderedItem milkFromPC = em.find(OrderedItem.class, milk.getId());

System.out.println(milkFromPC.getName() + " ordered by customer: " + 
                   milkFromPC.getOwner().getName());

// Changing the state of Owner entity through the OrderedItem
milkFromPC.getOwner().setName("Terrence");

stopTx();

Now, without @JoinColumn(updatable = false) in OrderedItem entity, the OrderedItem would be fetched from the PC, I'd access it's owner - a Customer - and would successfully modified its name. It wouldn't be a surprise because the Customer was also in managed state, so it had to be reflected in the database.

However, I assumed that updateable=false in @JoinColumn set on the One side of the relationship would prevent the UPDATE SQL statement from occurring. Unfortunately in the end I can see the name changed in the database (it's "Terrence" instead of "John"). I can also see the executed SQL UPDATE query:

[EL Fine]: 2011-11-30 23:41:27.941--ClientSession(16862753)--Connection(11024915)--Thread(Thread[main,5,main])--UPDATE CUSTOMER SET NAME = ? WHERE (ID = ?) bind => [Terrence, 1]

So - what does this updateable=false really do? Why do I need it? Does it protect only my foreign key from being changed? Is it like 'you can't change the entity but you can change the state of the entity'?