Java 在 jpa 中调用序列的下一个值
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Calling next value of a sequence in jpa
提问by Javi
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.
我有一个映射为实体的类以将其保存在数据库中。我有一个 id 字段作为主键，所以每次保存对象时，都会从序列“myClass_pk_seq”中检索 id 的值，代码如下所示。
@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?
我需要在“代码”属性中添加类似于 id 的内容。我需要一个序列，以便我可以分配给序列的下一个值进行编码（以保留该值，以防用户想要保留它但没有持久化数据）。我的意思是用户将看到该字段，如果他不知道要输入什么，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）。如何在不保留非主键字段的数据的情况下获取 JPA 中定义的序列的下一个值（并增加其值）？
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?
我只想有一个方法在与“代码”字段关联的序列上调用 nextval 并返回值。使用注释在 JPA 中执行此操作的最佳方法是什么？
Thanks.
谢谢。
采纳答案by Pascal Thivent
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?
我只想有一个方法在与“代码”字段关联的序列上调用 nextval 并返回值。使用注释在 JPA 中执行此操作的最佳方法是什么？
- 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.
- 当用户按下按钮时，使用原生 SQL 获取下一个序列值。手动创建序列或使用“假实体”让 JPA 为您创建它。
- 如果不想使用原生 SQL，则插入依赖于序列的实体并获取其 id。
Both solutions sounds a bit ugly. Maybe you could simply use a random generator like a UUID generator.
这两种解决方案听起来都有些难看。也许您可以简单地使用像 UUID 生成器这样的随机生成器。
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?
实际上，您没有提到任何有关code( 并且 JPA 注释没有表明它必须是唯一的) 的唯一性。你为什么不返回一个随机整数？
回答by razvanone
See another question/answers on the subject of using sequence defined elsewhere than id fields. You can create a fake entity with one field (of type Long id). Connect it to the sequence you defined in the DB. Then create a CrudRepository implementation for that entity and call its save() method with an empty instance of the fake entity object you defined. Hibernate will run for you a "select YOUR_SEQ.NEXTVAL from dual" query.
请参阅有关使用在 id fields 以外的其他地方定义的序列的主题的另一个问题/答案。您可以使用一个字段（Long id 类型）创建一个假实体。将其连接到您在 DB 中定义的序列。然后为该实体创建一个 CrudRepository 实现，并使用您定义的假实体对象的空实例调用其 save() 方法。Hibernate 将为您运行“select YOUR_SEQ.NEXTVAL from dual”查询。
