在JPA中保存对象之前如何知道ID [英] How do I know the id before saving an object in jpa

问题描述

我有一个新对象.我想知道ID，然后再保存.是否有可能?还是有另一种方法呢?我正在将jpa用作orm，将oracle用作数据库.

I have a new object. I want to know id before saving it. Is it possible? Or there is another way for this? I am using jpa as orm and oracle as database.

@Id
@Basic(optional = false)
@Column(name = "ID", nullable = false)
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "woTypeSeq")
private Long id;

我的实体中有一个代码字段.如果用户未在code字段中输入值，我想将代码设置为实体的ID.如果我保留实体，那么我当然可以获取ID并将代码值设置为ID，但这是额外的查询数据库.

I have a code field in my entity. If the user doesn't enter a value for the code field, I want to set code as entity's id. If I persist entity, of course i can get id and set code value as id, but this is extra query database.

我想做类似的事情

if(entity.getCode()==null) {
   entity.setCode(entity.getId);
   jpaDao.saveOrUpdate(entity);
}

推荐答案

经过长时间的研究，终于找到了解决方案.

After long researching about this, finally I found a solution.

实际上，如果在jpa中使用 序列生成器 ，则肯定无法在将实体的ID保存到数据库之前获取其ID，因为下一个ID将由数据库序列分配.

In fact, if you use sequence generator in jpa, certainly you cannot obtain entity's id before saving it in database, because next id will be assigned by database sequence.

如果使用 自定义生成器 ，则有一种获取ID的方法，可以在保存之前获取ID.这是简单的实现:

There is one way to obtain the id if you use a custom generator, you can get the id before saving. Here is simple implementation:

public class CustomGenerator extends IdentityGenerator implements Configurable {

    private IdentifierGenerator defaultGenerator;

    public Serializable generate(SessionImplementor session, Object object) throws HibernateException {
        Long idValue = (Long)defaultGenerator.generate(session, object);
        //idValue will be assigned your entity id
        return idValue;
    }

    @Override
    public void configure(Type type, Properties params, Dialect d) throws MappingException {
        DefaultIdentifierGeneratorFactory dd = new DefaultIdentifierGeneratorFactory();
        dd.setDialect(d);
        defaultGenerator = dd.createIdentifierGenerator("sequence", type, params);
    }
}

使用CustomGenerator作为id字段:

Using CustomGenerator for id field:

@Id
@Basic(optional = false)
@Column(name = "ID", nullable = false)
@GenericGenerator(name = "seq_id", strategy = "com.yoncabt.abys.core.listener.CustomGenerator", parameters = { @Parameter(name = "sequence", value = "II_FIRM_DOC_PRM_SEQ") })
@GeneratedValue(generator = "seq_id")
private Long id;

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