是否可以在没有 ID 的情况下使用自动增量编号? [英] Is it possible to have autoincrement number without it being an ID?

问题描述

我一直在谷歌搜索，发现唯一的方法就是使用

I keep googling and find that the only way is to use

@Id
@GeneratedValue(strategy = GenerationType.Identity)

但是我已经有了一个主键，我只需要另一个自动递增的字段.手动计算代码真的很难.

But I already have a primary key, I just need another field that auto increments. It's really hard to code it by manual calculation.

推荐答案

我看到以下选项:

1) 您可以使用 @Generated 注释.

1) You can use @Generated annotation.

您应该按以下方式声明一个表:

You should have a table declared in the following way:

create sequence TST_DATA_SEQ increment by 1 start with 1;

create table TST_DATA (
   ...
   dat_auto integer default nextval('TST_DATA_SEQ'),
   ...
);

和实体中的适当列:

   @Generated(value = GenerationTime.INSERT)
   @Column(name = "dat_auto", insertable = false, updatable = false)
   private Long auto;

请注意，根据文档:

标记为生成的属性还必须是不可插入和不可更新的.

Properties marked as generated must additionally be non-insertable and non-updateable.

因此，hibernate 将在刷新后进行额外查询以填充 auto 字段.

So, hibernate will make additional query to populate the auto field after flushing.

   Data data = new Data();
   // filling fields except data.auto
   session.persist(data);
   session.flush();

insert into TST_DATA (dat_name, dat_id) 
values (?, ?)

Hibernate: /* get generated state com.example.hibernate.Data */
  select data_.dat_auto as dat_auto_0_ 
  from TST_DATA data_ 
  where data_.dat_id=?

2) 您可以使用 @GeneratorType 注释.

2) You can use @GeneratorType annotation.

您应该有一个 hibernate ValueGenerator 的实现.您可以在下面看到的简单示例.

You should have an implementation of hibernate ValueGenerator. The simple example you can see below.

import java.math.BigInteger;
import org.hibernate.Session;
import org.hibernate.tuple.ValueGenerator;

public class MyGenerator implements ValueGenerator<Long> 
{
   public Long generateValue(Session session, Object owner)
   {
      return (
         (BigInteger) session
            .createNativeQuery("select nextval('TST_DATA_SEQ')")
            .getSingleResult()
         ).longValue();
   }
}

然后你可以像这样使用它:

And then you can use it like this:

   @GeneratorType(type = MyGenerator.class, when = GenerationTime.INSERT)
   @Column(name = "dat_auto")
   private Long auto;

在这种情况下，您不应像 n1 中要求的那样为列声明提供默认值.并且相应的实体字段不应具有 @Column(... insertable = false, updatable = false).每次持久化这个实体时，hibernate 都会生成查询:

In this case you should not provide the default value for the column declaration as was required in n1. and the appropriate entity field should not have @Column(... insertable = false, updatable = false). Each time when this entity will be persisted, hibernate generates query:

select nextval('TST_DATA_SEQ')

并将此值填充到 auto 字段.

and populate this value to the auto field.

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