hibernate one-to-many

同样的，班级和学生是一对多的关系，
目标，查询班级的时候，希望查询出班级学生列表！！

GradeClass.java  POJO代码

public class GradeClass {

	private Long gradeClassId;
	private String gradeClassName;
	private Set<User> users = new HashSet<User>();
         //...省略get和set方法
}

GradeClass.hbm.xml代码

<hibernate-mapping>
		<class name="com.supben.pojo.GradeClass" table="gradeclass">
				<id name="gradeClassId" type="java.lang.Long">
						<column length="20" name="gradeClassId" />
						<generator class="identity"></generator>
				</id>

				<property name="gradeClassName" type="java.lang.String">
						<column name="gradeClassName" />
				</property>
  <!--column="gradeClassId"指的是user（class属性指定）表中的gradeClassId字段 -->
				<set name="users" lazy="false" fetch="join">
						<key column="gradeClassId" />
						<one-to-many class="com.supben.pojo.User" />
				</set>

		</class>
</hibernate-mapping>

user.hbm.xml： 和平时配置没有任何的不同

<hibernate-mapping>
		<class name="com.supben.pojo.User" table="User">
				<id name="userId" type="java.lang.Long">
						<column length="20" name="userId" />
						<generator class="identity"></generator>
				</id>

				<property name="userName" type="java.lang.String">
						<column name="userName" />
				</property>
		</class>
</hibernate-mapping>

测试代码

public void testFind() {
		List<GradeClass> classList = gradeClassService.findAll();
		System.out.println("*************************************");
		Set<User> users = classList.get(0).getUsers();

		for (User user : users) {
			System.out.println(user.getUserName());
		}
	}

运行结果：

Hibernate: select this_.gradeClassId as gradeCla1_0_1_, this_.gradeClassName as gradeCla2_0_1_, users2_.gradeClassId as gradeCla3_3_, users2_.userId as userId3_, users2_.userId as userId1_0_, users2_.userName as userName1_0_, users2_.gradeClassId as gradeCla3_1_0_ from gradeclass this_ left outer join User users2_ on this_.gradeClassId=users2_.gradeClassId
*************************************
chenglong
supben


注：GradeClass.hbm.xml中set里面的column="gradeClassId"，对应的是user表里面的字段。 如同many-to-one一样，one-to-many默认的也是lazy="true" fetch="select"。（见前面一篇）。所以可以酌情配置成lazy="false" fetch="join"