Hibernate一对一外键关联实现方法实例

有连个实体如下：

CREATE   TABLE  `product` (
  `id`  varchar ( 50 )  NOT   NULL ,
  `name`  varchar ( 50 )  default   NULL ,
   PRIMARY   KEY   (`id`)
) ENGINE = InnoDB  DEFAULT  CHARSET = gb2312;

CREATE   TABLE  `product` (
  `id`  varchar ( 50 )  NOT   NULL ,
  `name`  varchar ( 50 )  default   NULL ,
   PRIMARY   KEY   (`id`)
) ENGINE = InnoDB  DEFAULT  CHARSET = gb2312;

 首先，建立这两个实体如下：

 

package  onetoone;

public   class  Image  ... {
   private String id;
   private String name;
   private Product product;

public Product getProduct() ...{
    return product;
}
public void setProduct(Product product) ...{
    this.product = product;
}
public String getId() ...{
    return id;
}
public void setId(String id) ...{
    this.id = id;
}
public String getName() ...{
    return name;
}
public void setName(String name) ...{
    this.name = name;
}
}


package  onetoone;

public   class  Product  ... {
    private String id;
    private String name;
    private Image image;
    public String getId() ...{
        return id;
    }
    public void setId(String id) ...{
        this.id = id;
    }
    public Image getImage() ...{
        return image;
    }
    public void setImage(Image image) ...{
        this.image = image;
    }
    public String getName() ...{
        return name;
    }
    public void setName(String name) ...{
        this.name = name;
    }

}

接下来建立两个持久类的mapping文件

 

<? xml version="1.0" encoding="utf-8" ?>
<! DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >
<!--  
    Mapping file autogenerated by MyEclipse - Hibernate Tools
-->
< hibernate-mapping  package ="onetoone" >  
   < class  name ="Product"  table ="product" >  
     < id  name ="id"  unsaved-value ="null" >
       < column  name ="id" ></ column >
       < generator  class ="uuid.hex" ></ generator >
     </ id >
     < property  name ="name"  column ="name" ></ property >
     < one-to-one  name ="image"
                class ="Image"
                fetch ="join"
                cascade ="all"
                property-ref ="product" >
     </ one-to-one >
   </ class >  

</ hibernate-mapping >

<? xml version="1.0" encoding="utf-8" ?>
<! DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >
<!--  
    Mapping file autogenerated by MyEclipse - Hibernate Tools
-->
< hibernate-mapping  package ="onetoone"    >  
   < class  name ="Image"  table ="image" >  
     < id  name ="id" >
       < generator  class ="uuid.hex" />
     </ id >
     < property  name ="name"  column ="name" ></ property >
      < many-to-one  name ="product"
                 class ="Product"
                 unique ="true"

                 column ="productid" >              
      </ many-to-one >
   </ class >  

</ hibernate-mapping >

测试代码：

 

     public   static   void  main(String[] args)  ... {
        Configuration cfg=new Configuration();
        cfg.configure();
        SessionFactory sf=cfg.buildSessionFactory();
        Session session=sf.openSession();
        Transaction t=session.beginTransaction();
        
//        Image image=new Image();
//        image.setName("image1");
//        
//        Product product=new Product();
//        product.setName("product1");
//        product.setImage(image);
//        image.setProduct(product);
//        session.save(product);
        

        Product p=(Product)session.get(Product.class, "1");
        System.out.println(p.getImage());

        t.commit();
        System.out.println("success");

    }

先在数据库中插入一个product的记录，id为1，image也插入一条记录，其productid为1，便于测试 查询代码

查询结果为
Hibernate: select product0_.id as id1_1_, product0_.name as name1_1_, image1_.id as id0_0_, image1_.name as name0_0_, image1_.productid as productid0_0_ from product product0_ left outer join image image1_ on product0_.id=image1_.productid where product0_.id=?
onetoone.Image@6754d6
success

有一点很重要，也是困扰笔者几个小时的地方，就是在主方（product）的配置文件中<one-to-one>中的
property-ref="product" 如果不加这个属性，Hibernate会使用image表的主键作为关联类的属性，结果HQL语句就会变成
Hibernate: select product0_.id as id1_1_, product0_.name as name1_1_, image1_.id as id0_0_, image1_.name as name0_0_, image1_.productid as productid0_0_ from product product0_ left outer join image image1_ on product0_.id=image1_.id where product0_.id=?

大家注意看，product0_.id=image1_.i，这明显是不对的，应该是product0_.id=image1_.productid ,这样才能正确关联上