@ManyToOne（updatable = false）-应该如何工作？ [英] @ManyToOne(updatable=false) - how it should work?

问题描述

我想在一个实体中具有只读功能。我知道在JPA 2.0中我们本身没有这样的功能。我以为我们可以使用 updateable = false，insertable = false 来实现它，但我不认为这是如何工作的。

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.

假设我有两个实体： OrderedItem 和 Customer ：

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
}

现在考虑以下代码：

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();

现在，没有 @JoinColumn（updatable = false）在 OrderedItem 实体中， OrderedItem 将从PC上获取，我可以访问它的所有者- Customer -并将成功修改其名称。这并不奇怪，因为 Customer 也处于托管状态，因此必须将其反映在数据库中。

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.

但是，我假设 @JoinColumn 中的 updateable = false 设置为关系的一侧将阻止UPDATE SQL语句的发生。不幸的是，最后我可以在数据库中看到名称更改（它是 Terrence而不是 John）。我还可以看到执行的SQL UPDATE查询：

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）-连接（11024915）-Thread（Thread [main，5，main]）-UPDATE
客户设置名称=？ WHERE（ID =？）bind => [Terrence，1]

[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]

所以-这是什么 updateable =错误真的吗？我为什么需要它？它仅保护我的外键不被更改吗？就像您不能更改实体，但可以更改实体的状态吗？

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'?

推荐答案

从< a href = http://docs.oracle.com/javaee/5/api/javax/persistence/JoinColumn.html rel = nofollow>文档

该列是否包含在持久性提供程序生成的SQL UPDATE语句中。

Whether the column is included in SQL UPDATE statements generated by the persistence provider.

就像您说的那样，它仅保护我的外键不被更改

So like you said, "it protects only my foreign key from being changed"

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