Hibernate 关联关系映射实例

双向多对一/一对多(many-to-one/one-to-many)

例子,多个学生对应一个班级,一个班级对应多个学生:

班级类,Grade.java:

public class Grade {

    private Integer id;
private String name;
private String description;
//学生集合
private Set<Student> students = new HashSet<Student>(); //省略get,set方法...... }

学生类,Student.java

public class Student {

    private Integer id;
private String name;
private String sex;
//班级引用
private Grade grade; //省略get,set方法....... }

班级类(一的一方)映射文件,Grade.hbm.xml:

<hibernate-mapping package="accp.hib">
<class name="Grade" table="Grade" >
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" type="string" />
<property name="description" type="string" column="description" />
<!-- 映射学生类集合 -->
<set name="students" inverse="true">
<key column="grade_id" />
<one-to-many class="Student"/>
</set>
</class>
</hibernate-mapping>

学生类(多的一方)映射文件,Student.hbm.xml:

<hibernate-mapping package="accp.hib">
<class name="Student" table="Student">
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" type="string" />
<property name="sex" type="string" column="sex" />
<!-- 映射班级类引用 -->
<many-to-one name="grade" class="Grade" column="grade_id" cascade="save-update"/>
</class>
</hibernate-mapping>

双向一对一(One-to-one)

例子,一个学生对应一个学生证,一个学生证对应一个学生:

学生类,Student.java

public class Student {
private Integer id;
private String name;
private String sex;
//学生证类 引用
private Paper paper; //省略get,set方法......
}

学生证类,Paper.java

public class Paper {

    private Integer id;
private String description;
//学生类引用
private Student student; //省略get,set方法.....
}

学生类(无外键一方)映射文件,Student.hbm.xml:

<hibernate-mapping package="accp.hib">
<class name="Student" table="Student">
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" type="string" />
<property name="sex" type="string" column="sex" />
<!-- 映射学生证类 -->
<one-to-one name="paper" class="Paper" />
</class>
</hibernate-mapping>

学生证类(有外键一方)映射文件,Paper.hbm.xml:

<hibernate-mapping package="accp.hib">
<class name="Paper" table="Paper" >
<id name="id" column="id">
<generator class="native" />
</id>
<property name="description" type="string" column="description" />
        <!-- 映射学生类 -->
<many-to-one name="student" column="student_id" class="Student" unique="true" cascade="save-update"/>
</class>
</hibernate-mapping>

要数据库中,外键会建立在写有 many-to-one 所映射的表中,也就是paper类所映射的表。

双向多对多(many-to-many)

例子,多个学生对应多门课程,多门课程也对应多个学生:

学生类,Student.java

public class Student {
private Integer id;
private String name;
private String sex;
//课程类集合
private Set<Course> courses = new HashSet<Course>(); //省略get,set方法.......
}

课程类,Course.java

public class Course {
private Integer id;
private String name;
//学生类集合
private Set<Student> students = new HashSet<Student>(); //省略get,set方法.......
}

学生类映射文件,Student.hbm.xml:

<hibernate-mapping package="accp.hib">
<class name="Student" table="Student">
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" type="string" />
<property name="sex" type="string" column="sex" />
<!-- 映射课程类集合 -->
<set name="courses" table="sc">
<key column="student_id" />
<many-to-many column="course_id" class="Course"/>
</set>
</class>
</hibernate-mapping>

课程类映射文件,Course.hbm.xml:

<hibernate-mapping package="accp.hib">
<class name="Course" table="Course" >
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" type="string" />
<!-- 映射学生类集合 -->
<set name="students" table="sc" inverse="true">
<key column="course_id" />
<many-to-many column="student_id" class="Student" />
</set>
</class>
</hibernate-mapping>

会新建一张名为 sc 的中间表,该表拥有student和course的外键,通过该中间表为student和course建立多对多关系。

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