hibernate入门使用系列 7-- annotation关系映射篇（中）

这次说说OneToMany和ManyToOne

 

我们的场景是 1个父亲n多个孩子

 

先来看看这中做法：

TestFather.java

package net.paoding.forum.domain;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class TestFather
{
    String          id;
    String          name;
    List<TestChild> children = new ArrayList<TestChild>();

    /**
     * @return the id
     */
    @Id
    public String getId()
    {
        return id;
    }

    /**
     * @param id the id to set
     */
    public void setId(String id)
    {
        this.id = id;
    }

    /**
     * @return the name
     */
    public String getName()
    {
        return name;
    }

    /**
     * @param name the name to set
     */
    public void setName(String name)
    {
        this.name = name;
    }

    /**
     * @return the children
     */
    @OneToMany
    public List<TestChild> getChildren()
    {
        return children;
    }

    /**
     * @param children the children to set
     */
    public void setChildren(List<TestChild> children)
    {
        this.children = children;
    }
}

 

 

TestChild.java

package net.paoding.forum.domain;

import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class TestChild
{
    String     id;
    String     name;

    /**
     * @return the id
     */
    @Id
    public String getId()
    {
        return id;
    }

    /**
     * @param id the id to set
     */
    public void setId(String id)
    {
        this.id = id;
    }

    /**
     * @return the name
     */
    public String getName()
    {
        return name;
    }

    /**
     * @param name the name to set
     */
    public void setName(String name)
    {
        this.name = name;
    }
}

 

这样子，好像就可以了。那么是的么？我们来看看hib产生的sql

drop table test_child cascade constraints;
drop table test_father cascade constraints;
drop table test_father_children cascade constraints;

create table test_child (
    id varchar2(255 char) not null,
    name varchar2(255 char),
    primary key (id)
);

create table test_father (
    id varchar2(255 char) not null,
    name varchar2(255 char),
    primary key (id)
);

create table test_father_children (
    null_id varchar2(255 char) not null,
    children_id varchar2(255 char) not null,
    unique (children_id)
);


alter table test_father_children 
    add constraint FK2E6E87D5901B7DBB 
    foreign key (null_id) 
    references test_father;

alter table test_father_children 
    add constraint FK2E6E87D58CD0E56B 
    foreign key (children_id) 
    references test_child;

alter table test_user 
    add constraint FKB9A96B58A237D846 
    foreign key (card_id) 
    references test_card;

 oh！ My God ！它居然又帮我们多创建了一个table。作为关联。太笨了。

 

很明显，这不是我们所需要的。

那我们该如何呢？

 

评论

3 楼 RyanPoy 2008-11-12  

由此我个人给出的结论是： 如果想通过hibernate的 po->ddl 这个功能，在ManyToOne的时候，一定要加上OneToMany配合使用。这样子，才能达到预想的效果。

2 楼 RyanPoy 2008-11-12  

接上
总不能放弃吧。再来。这次我们修改一下TestFather.java

package net.paoding.forum.domain;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class TestFather
{
    String          id;
    String          name;
    List<TestChild> children = new ArrayList<TestChild>();

    /**
     * @return the id
     */
    @Id
    public String getId()
    {
        return id;
    }

    /**
     * @param id the id to set
     */
    public void setId(String id)
    {
        this.id = id;
    }

    /**
     * @return the name
     */
    public String getName()
    {
        return name;
    }

    /**
     * @param name the name to set
     */
    public void setName(String name)
    {
        this.name = name;
    }

    /**
     * @return the children
     */
    @OneToMany(mappedBy="father")
    public List<TestChild> getChildren()
    {
        return children;
    }

    /**
     * @param children the children to set
     */
    public void setChildren(List<TestChild> children)
    {
        this.children = children;
    }
}

注意@OneToMany, 我们加上了 mappedBy="father", 这次看看sql吧：

   drop table test_child cascade constraints;
drop table test_father cascade constraints;

create table test_child (
        id varchar2(255 char) not null,
        name varchar2(255 char),
        father_id varchar2(255 char),
        primary key (id)
    );

create table test_father (
    id varchar2(255 char) not null,
    name varchar2(255 char),
    primary key (id)
);

alter table test_child 
        add constraint FK7A81672F268831C6 
        foreign key (father_id) 
        references test_father;

alter table test_user 
    add constraint FKB9A96B58A237D846 
    foreign key (card_id) 
    references test_card;

我的天啊！终于对了。累死了。这么说，难道就没有实现单向的么？只能通过xml配置？hib啊，你还是比较难用。

1 楼 RyanPoy 2008-11-12  

接上。
于是，我们修改代码：
TestChild.java

package net.paoding.forum.domain;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
public class TestChild
{
    String     id;
    String     name;
    TestFather father;

    /**
     * @return the id
     */
    @Id
    public String getId()
    {
        return id;
    }

    /**
     * @param id the id to set
     */
    public void setId(String id)
    {
        this.id = id;
    }

    /**
     * @return the name
     */
    public String getName()
    {
        return name;
    }

    /**
     * @param name the name to set
     */
    public void setName(String name)
    {
        this.name = name;
    }

    /**
     * @return the father
     */
    @ManyToOne
    public TestFather getFather()
    {
        return father;
    }

    /**
     * @param father the father to set
     */
    public void setFather(TestFather father)
    {
        this.father = father;
    }
}

那么，这次产生的sql呢？

drop table test_child cascade constraints;
drop table test_father cascade constraints;
drop table test_father_children cascade constraints;

create table test_child (
    id varchar2(255 char) not null,
    name varchar2(255 char),
    father_id varchar2(255 char),
    primary key (id)
);

create table test_father (
    id varchar2(255 char) not null,
    name varchar2(255 char),
    primary key (id)
);

create table test_father_children (
    null_id varchar2(255 char) not null,
    children_id varchar2(255 char) not null,
    unique (children_id)
);	

alter table test_child 
    add constraint FK7A81672F268831C6 
    foreign key (father_id) 
    references test_father;

alter table test_father_children 
    add constraint FK2E6E87D5901B7DBB 
    foreign key (null_id) 
    references test_father;

alter table test_father_children 
    add constraint FK2E6E87D58CD0E56B 
    foreign key (children_id) 
    references test_child;

alter table test_user 
    add constraint FKB9A96B58A237D846 
    foreign key (card_id) 
    references test_card;

唉！失望了！？hib真TMD愚蠢。