JPA - 以编程方式通过序列递增数字字段

Question

我有一个JPA 2 Web应用程序(Struts 2,Hibernate 4仅作为JPA实现).

当前的要求是将(非id)数字顺序字段(仅对某些行填充)添加到现有实体.根据特定条件插入新行时,我需要将新字段设置为its highest value + 1或NULL.

例如:

ID     NEW_FIELD     DESCRIPTION
--------------------------------
1          1           bla bla
2                      bla bla       <--- unmatched: not needed here
3                      bla bla       <--- unmatched: not needed here
4          2           bla bla
5          3           bla bla
6          4           bla bla
7                      bla bla       <--- unmatched: not needed here
8          5           bla bla
9                      bla bla       <--- unmatched: not needed here
10         6           bla bla

在旧的SQL中,它将是这样的:

INSERT INTO myTable (
    id, 
    new_field, 
    description
) VALUES (
    myIdSequence.nextVal, 
    (CASE myCondition
        WHEN true 
        THEN myNewFieldSequence.nextVal
        ELSE NULL
    END),
    'Lorem Ipsum and so on....'
)

但我不知道如何用JPA 2实现它.

我知道我可以定义回调方法,但是Eval 2.0 Eval的JSR-000317持久性规范不鼓励其中的一些特定操作:

3.5实体监听器和回调方法
<sub>- 生命周期回调可以调用JNDI,JDBC,JMS和企业bean.
- 通常,可移植应用程序的生命周期方法不应调用EntityManager或Query操作,访问其他实体实例或修改同一持久性上下文中的关系.^[43]阿生命周期回调方法可以修改在其上调用实体的非关系状态.</sub>

^[43] _{这样的操作的语义可以在以后的本说明书中的释放标准化.}

总结一下,是JDBC(!)和EJB,不是EntityManager和其他实体.

---

编辑

我正在努力实现@anttix答案中描述的解决方案,但我遇到了一些问题,所以请在我错的地方纠正我.

表

MyTable
-------------------------
ID            number (PK)
NEW_FIELD     number
DESCRIPTION   text

主要实体

@Entity
@Table(name="MyTable")
public class MyEntity implements Serializable {

    @Id
    @SequenceGenerator(name="seq_id", sequenceName="seq_id", allocationSize=1)
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="seq_id")
    private Long id;

    @OneToOne(cascade= CascadeType.PERSIST) 
    private FooSequence newField;

    private String description

    /* Getters and Setters */
}

子实体

@Entity
public class FooSequence {

    @Id
    @SequenceGenerator(name="seq_foo", sequenceName="seq_foo", allocationSize=1)
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="seq_foo")
    private Long value;

    /* Getter and Setter */
}

DAO

myEntity.setNewField(new FooSequence());
entityManager.persist(myEntity);

例外

引起:javax.transaction.RollbackException:ARJUNA016053:无法提交事务.

[...]

引起:javax.persistence.PersistenceException:org.hibernate.exception.SQLGrammarException:错误:关系"new_field"不存在

[...]

引起:org.hibernate.exception.SQLGrammarException:错误:关系"new_field"不存在

[...]

引起:org.postgresql.util.PSQLException:错误:关系"new_field"不存在

我究竟做错了什么 ？我对JPA 2很新,我从未使用过与物理表无关的实体......这种方法对我来说是全新的.

我想我需要把@Column定义的地方:JPA怎么可能知道newField列(通过映射ImprovedNamingStrategy到new_field数据库上)通过获取value的财产FooSequence实体？

缺少一些拼图.

---

编辑

正如评论中所说,这是persistence.xml:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" 
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
                     http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">

    <persistence-unit name="MyService"  transaction-type="JTA">

        <jta-data-source>java:jboss/datasources/myDS</jta-data-source>      

        <properties>             

            <property name="hibernate.dialect" 
                     value="org.hibernate.dialect.PostgreSQLDialect" />

            <property name="hibernate.ejb.naming_strategy" 
                     value="org.hibernate.cfg.ImprovedNamingStrategy"/>

            <property name="hibernate.query.substitutions" 
                     value="true 'Y', false 'N'"/>           

         <property name="hibernate.show_sql" value="true" />
         <property name="format_sql"         value="true" />
         <property name="use_sql_comments"   value="true" />

        </properties>

    </persistence-unit>
</persistence>

Answer 1

一种可能的解决方案是使用具有自己的表的单独实体，该实体将仅封装新字段并与该实体进行 OneToOne 映射。仅当遇到需要附加序列号的对象时，您才会实例化新实体。然后，您可以使用任何生成器策略来填充它。

@Entity
public class FooSequence {
    @Id
    @GeneratedValue(...)
    private Long value;
}

@Entity 
public class Whatever {
    @OneToOne(...)
    private FooSequnce newColumn;
}

看：

• Hibernate JPA 序列（非 Id）
• https://forum.hibernate.org/viewtopic.php?p=2405140

一个 gradle 1.11 可运行的 SSCCE（使用 Spring Boot）：

src/main/java/JpaMultikeyDemo.java

import java.util.List;
import javax.persistence.*;
import lombok.Data;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.annotation.Configuration;
import org.springframework.transaction.annotation.EnableTransactionManagement;
import org.springframework.transaction.annotation.Transactional;

@Configuration
@EnableTransactionManagement
@EnableAutoConfiguration
public class JpaMultikeyDemo {
    @Entity @Data
    public static class FooSequence {
        @Id @GeneratedValue private Long value;
    }

    @Entity @Data
    public static class FooEntity {
        @Id @GeneratedValue private Long id;
        @OneToOne           private FooSequence sequence;
    }

    @PersistenceContext
    EntityManager em;

    @Transactional
    public void runInserts() {
        // Create ten objects, half with a sequence value
        for(int i = 0; i < 10; i++) {
            FooEntity e1 = new FooEntity();
            if(i % 2 == 0) {
                FooSequence s1 = new FooSequence();
                em.persist(s1);
                e1.setSequence(s1);
            }
            em.persist(e1);
        }
    }

    public void showAll() {
        String q = "SELECT e FROM JpaMultikeyDemo$FooEntity e";
        for(FooEntity e: em.createQuery(q, FooEntity.class).getResultList())
            System.out.println(e);
    }

    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(JpaMultikeyDemo.class);
        context.getBean(JpaMultikeyDemo.class).runInserts();
        context.getBean(JpaMultikeyDemo.class).showAll();
        context.close();
    }
}

构建.gradle

apply plugin: 'java'
defaultTasks 'execute'

repositories {
    mavenCentral()
    maven { url "http://repo.spring.io/libs-milestone" }
}

dependencies {
    compile "org.springframework.boot:spring-boot-starter-data-jpa:1.0.0.RC5"
    compile "org.projectlombok:lombok:1.12.6"
    compile "com.h2database:h2:1.3.175"
}

task execute(type:JavaExec) {
    main = "JpaMultikeyDemo"
    classpath = sourceSets.main.runtimeClasspath
}

另请参阅：http ://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#boot-features-configure-datasource