这几天，在多队多的中间表的操作上遇到了点麻烦，总是不能删除和更新中间表，因为，我使用的是一队多的关联，
映射的是中间表，所以，我虽然操作了set,但是总是不能反映到中间表上去。

后来使用如下的方法来解决，看如下的例子：

（1）三个表

CREATE TABLE `poi` (
  `poi_id` INTEGER(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `poi_name` VARCHAR(45) COLLATE latin1_swedish_ci NOT NULL DEFAULT '',
  PRIMARY KEY (`poi_id`),
  UNIQUE KEY `poi_name` (`poi_name`)

)ENGINE=InnoDB
AUTO_INCREMENT=15 CHARACTER SET 'latin1' COLLATE 'latin1_swedish_ci'
COMMENT='InnoDB free: 3072 kB';

CREATE TABLE `user` (
  `user_id` INTEGER(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `username` VARCHAR(45) COLLATE latin1_swedish_ci NOT NULL DEFAULT '',
  `password` VARCHAR(45) COLLATE latin1_swedish_ci NOT NULL DEFAULT '',
  PRIMARY KEY (`user_id`),
  UNIQUE KEY `username` (`username`)

)ENGINE=InnoDB
AUTO_INCREMENT=16 CHARACTER SET 'latin1' COLLATE 'latin1_swedish_ci'
COMMENT='InnoDB free: 3072 kB';

关联表

CREATE TABLE `user_poi` (
  `user_id` INTEGER(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `poi_id` INTEGER(10) UNSIGNED NOT NULL,
  KEY `FK_user_pois_poiid` (`poi_id`),
  KEY `FK_user_pois_userid` (`user_id`),
  CONSTRAINT `FK_user_pois_poiid` FOREIGN KEY (`poi_id`) REFERENCES `poi` (`poi_id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `FK_user_pois_userid` FOREIGN KEY (`user_id`) REFERENCES `user` (`user_id`) ON DELETE CASCADE ON UPDATE CASCADE

)ENGINE=InnoDB
AUTO_INCREMENT=3 CHARACTER SET 'latin1' COLLATE 'latin1_swedish_ci'
COMMENT='InnoDB free: 3072 kB; (`poi_id`) REFER `demo/poi`(`poi_id`) ON UPDATE CASCADE; (';

（2）POJO


/**
 * @hibernate.class table="user"
 */
public class User implements java.io.Serializable {

    // Fields

    private Integer userid;
    private String username;
    private String password;

    /**
     * Poi set
     */
    private Set<Poi> pois = new HashSet();
   
   
   
    // Constructors

    /**
     * @hibernate.set table="user_poi" cascade="all"
     * @hibernate.collection-key column="user_id"
     * @hibernate.collection-many-to-many column="poi_id" class="org.zy.pro.pd.dao.Poi"
     */
    public Set getPois() {
        return pois;
    }

    public void setPois(Set pois) {
        this.pois = pois;
    }

    public User() {
    }

    /** full constructor */
    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }

    // Property accessors


    /**
     * @hibernate.id column="user_id" generator-class="increment"
     */
    public Integer getUserid() {
        return this.userid;
    }

    public void setUserid(Integer userid) {
        this.userid = userid;
    }

    /**
     * @hibernate.property column="username"
     * @return
     */
    public String getUsername() {
        return this.username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    /**
     * @hibernate.property column="password"
     * @return
     */
    public String getPassword() {
        return this.password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

}

/**
 * @hibernate.class table="poi"
 */
public class Poi  implements java.io.Serializable {


    // Fields   

     private Integer poiId;
     private String poiName;


    // Constructors

    /** default constructor */
    public Poi() {
    }

    /** minimal constructor */
    public Poi(String poiName) {
        this.poiName = poiName;
    }


    /**
     * @hibernate.id column="poi_id" generator-class="increment"
     */
    public Integer getPoiId() {
        return this.poiId;
    }
   
    public void setPoiId(Integer poiId) {
        this.poiId = poiId;
    }

    /**
     * @hibernate.property column="poi_name"
     * @return
     */
    public String getPoiName() {
        return this.poiName;
    }
   
    public void setPoiName(String poiName) {
        this.poiName = poiName;
    }
}


（3）hibernate mapping

利用xDolect根据bean生成映射

Poi.hbm.xml

<?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">

<hibernate-mapping
>
    <class
        name="org.zy.pro.pd.dao.Poi"
        table="poi"
    >

        <id
            name="poiId"
            column="poi_id"
            type="java.lang.Integer"
        >
            <generator class="increment">
    
            </generator>
        </id>

        <property
            name="poiName"
            type="java.lang.String"
            update="true"
            insert="true"
            column="poi_name"
        />

    </class>

</hibernate-mapping>

User.hbm.xml

<?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">

<hibernate-mapping
>
    <class
        name="org.zy.pro.pd.dao.User"
        table="user"
    >

        <id
            name="userid"
            column="user_id"
            type="java.lang.Integer"
        >
            <generator class="increment">

            </generator>
        </id>

        <set
            name="pois"
            table="user_poi"
            lazy="false"
            cascade="all"
            sort="unsorted"
        >

            <key
                column="user_id"
            >
            </key>

            <many-to-many
                class="org.zy.pro.pd.dao.Poi"
                column="poi_id"
                outer-join="auto"
             />

        </set>

        <property
            name="username"
            type="java.lang.String"
            update="true"
            insert="true"
            column="username"
        />

        <property
            name="password"
            type="java.lang.String"
            update="true"
            insert="true"
            column="password"
        />



    </class>

</hibernate-mapping>

（4）关联添加
        UserDAO ud = new UserDAO();
        List l = ud.findAll();
        assertNotNull(l);
       
        User u = ud.findById(2);
        assertNotNull(u);
       
        Set pois = u.getPois();
       
        Poi p = new Poi("Moc"+new java.util.Random().nextInt(1000000));
        System.out.print(p.getPoiId());
       
       
        pois.add(p);

//关联删除
//        u.setPois(null);
       
        Poi pp = pd.findById(12);
        pois.remove(pp);
       
       
        ud.begin();
        ud.merge(u);
        ud.commit();


// 关联更新也可以，只要改变set就可以了。



使用总结
［1］使用多对多关系
    在映射的是时候要使用many-to-many
    所以，在user的set集合里面直接存放的是Poi的对象，而不是user_poi表的对象
［2］reverse : false
    允许反转操作
［3］casecade : all
    只要允许级联操作才可以实现级联的删除和更新

如果使用一对多，就只能更新中间表的普通字段，不能更新主键字段，也不能删除中间表记录
所以只能维护中间表的关联,但是不能更新中间表的信息。





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