Hibernate關聯關係映射—–單向一對多配置

     這裡舉了皇帝和大臣的例子，天子只有一個，而諸侯多的不勝計數。

實體：

package uni.one2many;import java.util.HashSet;import java.util.Set;public class Emperor {private int id;private String name;private Set<Minister> ministers = new HashSet<Minister>();public Emperor() {}public Emperor(int id, String name, Set<Minister> ministers) {super();this.id = id;this.name = name;this.ministers = ministers;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Set<Minister> getMinisters() {return ministers;}public void setMinisters(Set<Minister> ministers) {this.ministers = ministers;}}package uni.one2many;public class Minister {private int id;private String name;public Minister() {}public Minister(int id, String name) {super();this.id = id;this.name = name;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}}

　　

hbm設定檔：

<hibernate-mapping><class name="uni.one2many.Emperor"><id name="id" column="did"><generator class="native" /></id><property name="name" type="string" column="name"></property><!-- confirure the uniredirection association --><set name="ministers" ><key column="id" not-null="true"></key><one-to-many class="uni.one2many.Minister"/></set></class></hibernate-mapping><hibernate-mapping><class name="uni.one2many.Minister"><id name="id" column="did"><generator class="native" /></id><property name="name" type="string" column="name"></property></class></hibernate-mapping>

　　

測試檔案：

public class Test {@org.junit.Testpublic void testAdd() {SessionFactory sf = HibernateUtil.getSessionFactory();Session session = sf.getCurrentSession();session.beginTransaction();Minister m1 = new Minister();m1.setName("廉頗");Minister m2 = new Minister();m2.setName("藺相如");Emperor emperor = new Emperor();emperor.setName("齊威王");emperor.getMinisters().add(m1);emperor.getMinisters().add(m2);session.save(emperor);session.save(m1);session.save(m2);session.beginTransaction().commit();}}

　　

測試結果：

ibernate: insert into Emperor (name) values (?)Hibernate: insert into Minister (name, id) values (?, ?)Hibernate: insert into Minister (name, id) values (?, ?)Hibernate: update Minister set id=? where did=?Hibernate: update Minister set id=? where did=?

　　

這種映射關係在儲存的過程中需要先將一的（主控方）一方先加以儲存，否則會出現異常。我們可以對這種異常加以分析，從這種關係產生的表結構可以看出，在Minister的表中有一個指向Emperor的外鍵，由此可以推測出，如果這個外鍵有了非空約束，那麼再儲存Minister的過程中必然會出現引用為空白的異常。