hibernate：表格没有id时

有些表没有定义id或主键，只有一个unique约束，你又无权修表。

但Hibernate说，一定要id，怎么办？

这时，可以把unique约束相关的field定义为id，一个类里，可以有多个@Id 。

这样可以省去写@EmbeddedId的麻烦。

下面是hibernate文档摘录：

http://docs.jboss.org/hibernate/stable/annotations/reference/en/html_single/

---------------------2.2.3.2.2. Multiple @Id properties

Another, arguably more natural, approach is to place @Id on multiple properties of my entity. This approach is only supported by Hibernate but does not require an extra embeddable component.

@Entity
class Customer implements Serializable {
  @Id @OneToOne
  @JoinColumns({
    @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
    @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
  })
  User user;
  
  @Id String customerNumber;

  boolean preferredCustomer;
}

 In this case Customer being it's own identifier representation, it must implement Serializable.

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