关联映射:
    1.关于n:1  使用emp和dept进行举例
     单项多对一:
     emp:
     entity:+private Dept dept;
     hbm.xml:+<many-to-one name="dept" column="deptno" class="com.entity.Dept" cascade="save-update" />
     dept无需修改
    
     双向一对多:
     emp:
     entity:+private Dept dept;
     hbm.xml:+<many-to-one name="dept" column="deptno" class="com.entity.Dept" cascade="save-update" />
     dept:
     entity:+private List<Emp> emps;
     hbm.xml:+<bag name="emps" inverse="true" cascade="all">
                 <key>
                     <column name="DEPTNO" precision="2" scale="0" />
                 </key>
                 <one-to-many class="com.entity.Emp" />
             </bag>
     关于inverse和cascade的设置
     inverse:反转  默认inverse="false"表示主动方  一般将多方设置为主动方
     cascade:级联  none save-update delete all 一般对一方设置级联
    2.多对多:使用Student表和Course表  中间需要一个StuCourse学生课程表
     单向多对多:
     Student:
     entity:+private List<Course> courses = new ArrayList<Course>();
     hbm.xml:
     <bag name="courses" table="STUCOURSE" inverse="false" cascade="save-update">
                <key>
                    <column name="STUNO" precision="4" scale="0" />
                </key>
                <many-to-many class="com.manytomany.entity.Course" >
                 <column name="COURSEID" precision="4" scale="0" />
                </many-to-many>
             </bag>
     Course:无需修改
     注意事项:关系表中只保存两列外键,设置inverse="false",在设置级联时需要设置cascade="save-update"
     双向多对多:
     Student:
     entity:+private List<Course> courses = new ArrayList<Course>();
     hbm.xml:
     <bag name="courses" table="STUCOURSE" inverse="false" cascade="save-update">
                <key>
                    <column name="STUNO" precision="4" scale="0" />
                </key>
                <many-to-many class="com.manytomany.entity.Course" >
                 <column name="COURSEID" precision="4" scale="0" />
                </many-to-many>
             </bag>
     Course:
     entity:+private List<Student> students = new ArrayList<Student>();
     hbm.xml:
     <bag name="students" table="STUCOURSE" inverse="false" cascade="save-update">
                <key>
                    <column name="COURSEID" precision="4" scale="0" />
                </key>
                <many-to-many class="com.manytomany02.entity.Student" >
                 <column name="STUNO" precision="4" scale="0" />
                </many-to-many>
            </bag>
     注意事项:关系表中只保存两列外键,设置inverse="false",在设置级联时需要设置cascade="save-update"
    3.一对一:Employee员工表和Passport员工信息表
 主键关联:
     Employee:
     entity:+private Passport passport;
     hbm.xml:
     <one-to-one name="passport"  class="com.entity.Passport" cascade="all" />
     Passport:
     entity:+private Employee employee;
     hbm.xml:
     <generator class="foreign" >
     <param name="property">employee</param>
     </generator>
     <one-to-one name="employee" class="com.entity.Employee" />
     注意事项:主键生成策略选择foreign
     唯一外键:
     Employee:
     entity:+private Passport passport;
     hbm.xml:
     <one-to-one name="passport" class="com.entity.Passport" property-ref="employee"  cascade="all" />
     Passport:
     entity:+private Employee employee;
     hbm.xml:
     <many-to-one name="employee" column="empno" class="com.entity.Employee" unique="true">