Hibernate关系映射（N--N）多对多
（1）N--N单向（多对多单向）? 多对多的关系，一定要找个中间表进行管理
第一步：类 对象之间的关系映射
?
t_teacher(id,name,age)? --教师表
t_student(id,name,age)? --学生表
t_teacher_student(t_id,s_id)? --教师学时关系表
?
public class Teacher {
??? private int id;
??? private String name;
??? private int age;
??? private Set<Student> students;
}
?
public class Student {
??? private int id;
??? private String name;
??? private int age;
}
?
第二步：xml的配置
??? <class name="Teacher" table="t_teacher">
??? ??? <id name="id" column="id">
??? ??? ??? <generator unique="true" not-null="true" column="name" length="32"/>??? ???
??? ??? <property name="age" column="age" length="32"/>
??? ???
??? ??? <set name="students" table="t_teacher_student" cascade="save-update">
??? ??? ??? <key column="t_id"></key>
??? ??? ??? <many-to-many column="s_id"></many-to-many>
??? ??? </set>?? ???
??? </class>
?
??? <class name="Student" table="t_student">
??? ??? <id name="id" column="id">
??? ??? ??? <generator unique="true" not-null="true" column="name" length="32"/>
??? ???? <property name="age" column="age" length="32"/>? ???
??? </class>
?
?
（1）N--N双向（多对多双向）? 多对多的关系，一定要找个中间表进行管理
第一步：类 对象之间的关系映射
t_teacher(id,name,age)? --教师表
t_student(id,name,age)? --学生表
t_teacher_student(t_id,s_id)? --教师学时关系表
?
?
public class Teacher {
??? private int id;
??? private String name;
??? private int age;
??? private Set<Student> students;
}
?
public class Student {
??? private int id;
??? private String name;
??? private int age;
? ? private Set<Teacher> teachers; // 增加了Teacher的集合
}
?
第二步：xml的配置（多对多的双向，两边写法一样的）
??? <class name="Student" table="t_student">
??? ??? <id name="id" column="id">
??? ??? ??? <generator unique="true" not-null="true" column="name" length="32"/>
??? ??? <property name="age" column="age" length="32"/>
??? ??? -- 增加了set配置
??? ??? <set name="teachers" table="t_teacher_student" cascade="save-update">
??? ??? ??? <key column="s_id"></key>
??? ??? ??? <many-to-many column="t_id"></many-to-many>
??? ??? </set>
??? </class>
?
??? <class name="Teacher" table="t_teacher">
??? ??? <id name="id" column="id">
??? ??? ??? <generator unique="true" not-null="true" column="name" length="32"/>
??? ??? <property name="age" column="age" length="32"/>
??? ???
??? ??? <set name="students" table="t_teacher_student" cascade="save-update">
??? ??? ??? <key column="t_id"></key>
??? ??? ??? <many-to-many column="s_id"></many-to-many>
??? ??? </set>
??? </class>
