jinfeng_wang

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 2.0//EN"
          "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
  <class name="com.oreilly.hh.Album" table="ALBUM">
    <meta attribute="class-description">
      Represents an album in the music database, an organized list of tracks.
      @author Jim Elliott (with help from Hibernate)
    </meta>

    <id name="id" type="int" column="ALBUM_ID">
      <meta attribute="scope-set">protected</meta>
      <generator class="native"/>
    </id>

    <property name="title" type="string">
      <meta attribute="use-in-tostring">true</meta>
      <column name="TITLE" not-null="true" index="ALBUM_TITLE"/>
    </property>

    <property name="numDiscs" type="integer" not-null="true"/>

    <set name="artists" table="ALBUM_ARTISTS">
      <key column="ALBUM_ID"/>
      <many-to-many class="com.oreilly.hh.Artist" column="ARTIST_ID"/>
    </set>

    <set name="comments" table="ALBUM_COMMENTS">
      <key column="ALBUM_ID"/>
      <element column="COMMENT" type="string"/>
    </set>

    <list name="tracks" table="ALBUM_TRACKS" cascade="all">
      <meta attribute="use-in-tostring">true</meta>
      <key column="ALBUM_ID"/>
      <index column="POS"/>
      <composite-element class="com.oreilly.hh.AlbumTrack">
        <many-to-one name="track" class="com.oreilly.hh.Track" cascade="all">
          <meta attribute="use-in-tostring">true</meta>
          <column name="TRACK_ID"/>
        </many-to-one>
        <property name="disc" type="integer" not-null="true"/>
        <property name="positionOnDisc" type="integer" not-null="true"/>
      </composite-element>
    </list>

    <property name="added" type="date">
      <meta attribute="field-description">When the album was created</meta>
    </property>

  </class>
</hibernate-mapping>

The cascade attribute tells Hibernate that you want operations performed on a 'parent' object to be transitively applied to its 'child' or 'dependent' objects. It's applicable to all forms of collections and associations. There are several possible values to choose among. The most common are none (the default), save-update, delete, and all (which combines save-update and delete). You can also change the default from none to save-update throughout your entire mapping document by supplying a default-cascade attribute in the hibernate-mapping tag itself.

級聯(lián)（cascade）屬性告訴hibernate：所有對父對象的操作都將付諸于子對象和依賴對象上。級聯(lián)可以應(yīng)用于各種形式的集合和關(guān)聯(lián)中。這里級聯(lián)的具體取值可以有多個，最常用的有：none(默認(rèn)值)，save-update,delete和all（其中包含了save-update和delete）。你可以在hibernate-mappiong tag中通過設(shè)置default-casade屬性，修改這里的casade的默認(rèn)值，例如修改其默認(rèn)值為save-update。

In our example, we want the tracks owned by an album to be automatically managed by the album, so that when we delete the album, its tracks are deleted. Note that we need to apply the cascade attribute both to the tracks collection and its constituent track element to achieve this. Also, by using a cascade value of all, we eliminate the need to explicitly save any Track objects we create for the album—the addAlbumTrack() method of Example 5-7 no longer needs the line:

   session.save(track);

在我們的例子中，我們希望track完全由album自動掌管，當(dāng)我們刪除album時，它的track也會被刪除。注意，這里我們需要在兩處設(shè)置casade屬性實現(xiàn)此任務(wù)：tracks集合和它的組成元素track。同樣，由于我們講casade設(shè)置為all，那么不再需要顯式的保存為album創(chuàng)建的track對象，5-7例子中的addAlbumTrack方法不再需要調(diào)用session.save(track)方法。

By telling Hibernate that it's fully responsible for the relationship between an album and its track, we enable it to persist tracks when they're added to the album as well as delete them when the album itself is deleted.