在jpa中调用序列的下一个值

问题描述

我有一个映射为实体的类将其保存在数据库中。我有一个id字段作为主键，因此每次持久对象时，id的值都会从序列myClass_pk_seq中检索，类似于下面的代码。

  @Entity 
 @Table（name =myObjects）
 public class MyClass {
 @Id 
 @GeneratedValue（strategy = GenerationType .SEQUENCE，generator =sequence）
 @SequenceGenerator（name =sequence，sequenceName =myClass_pk_seq，allocationSize = 1）
 @Column（name =myClassId）
 private整数ID; 
 
私人整数代码; 
 
 
 ... 
} 
  

我需要在code属性中做类似于id的东西。我需要一个序列，以便我可以指定序列的下一个值（以保留该值，以防用户想要保留该值但不保留数据）。我的意思是用户会看到该字段，如果他不知道该输入什么内容，他可以按下按钮并在屏幕上接收下一个可能的值（他可以接受或不接受）。我怎样才能获得在JPA中定义的序列的下一个值（并增加它的值），而不需要保存非主键字段的数据？

我只想拥有调用与代码字段相关的序列的nextval并返回该值的方法。

谢谢。

解决方案

在JPA中使用注释执行此操作的最佳方式是什么？ >

• 使用原生SQL获取下一个序列当用户按下按钮时的值。要么手动创建序列，要么使用假实体让JPA为您创建它。


• 如果您不想使用原生SQL，请插入依赖序列的实体并获得它的ID。

这两个解决方案听起来有点难看。也许你可以简单地使用像UUID生成器那样的随机生成器。实际上，你没有提及任何有关代码的唯一性（并且JPA注释不包含任何关于代码表明它必须是唯一的）。你为什么不返回一个随机int？

I have a class mapped as an Entity to persist it in a database. I have an id field as the primary key so every time the object is persisted the value of the id is retrieved from the sequence "myClass_pk_seq", a code like the following one.

@Entity
@Table(name="myObjects")
public class MyClass {
    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="sequence")
    @SequenceGenerator(name="sequence", sequenceName="myClass_pk_seq", allocationSize=1)
    @Column(name="myClassId")
    private Integer id;

    private Integer code;


    ...
}

I need to make in "code" attribute something similar to id. I need to have a sequence so I can assign to code the next value of the sequence (to reserve that value in case the user wants to reserve it but without persisting data). I mean the user will see the field, if he doesn't know what to enter he can push a button and receieve in the screen the next possible value (and he can or not accept it). How can I get the next value of a sequence defined in JPA (and increment its value) without persisting the data for a non primary key field?

I just want to have a method which call nextval on a sequence associated with "code" field, and returns the value. What's the best way to do it in JPA with annotations?

Thanks.

解决方案

• Use native SQL to get the next sequence value when the user pushes the button. Either create the sequence manually or use a "fake entity" to have JPA create it for you.
• If you don't want to use native SQL, insert an entity relying on the sequence and gets its id.

Both solutions sounds a bit ugly. Maybe you could simply use a random generator like a UUID generator.

Actually, you didn't mention anything about the uniqueness of the code (and the JPA annotations don't show it must be unique). Why don't you return a random int?

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