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">
