单向1:1映射

相像一下一个员工只属于一个部门，站在员工的立场，就成了一对一了。

其实N:1与1:1的差别就是N与1的不同，其实把N变成1不就成了1:1了吗？怎么变呢？就是在之前的<many-to-one>中加入一个unique="true",其实这个unique属于数据库层次的限制，它将会限制department的id唯一，这样就直接导致了一个员工只能属于一个部门。

对于单向1:1无中间表的映射，不变的代码请参考单向N:1无中间表映射

更改的部分：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="com.longtech.hibernate.domain.Employee"
        table="Employee"> 
        <id name="id" column="id" type="java.lang.Long" >
            <meta attribute="field-description">唯一标识</meta>
            <generator class="native"/>
        </id>
        <property name="name" type="java.lang.String" column="name" length="20">
            <meta attribute="field-description">员工名称</meta>
        </property>
        <many-to-one name="department" column="dept_id" cascade="all" unique="true"></many-to-one> 
<!--         <join table="employ_depart">
            <key column="empid"></key>
            <many-to-one name="department" cascade="all" column="deptid"></many-to-one>
        </join> -->
    </class>
</hibernate-mapping>

单向1:1有中间表映射和单向N:1有中间表映射配置类似，改变的有

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="com.longtech.hibernate.domain.Employee"
        table="Employee"> 
        <id name="id" column="id" type="java.lang.Long" >
            <meta attribute="field-description">唯一标识</meta>
            <generator class="native"/>
        </id>
        <property name="name" type="java.lang.String" column="name" length="20">
            <meta attribute="field-description">员工名称</meta>
        </property>
        <!-- <many-to-one name="department" column="dept_id" cascade="all"></many-to-one> -->
        <join table="employ_depart">
            <key column="empid"></key>
            <many-to-one name="department" cascade="all" column="deptid" unique="true"></many-to-one>
        </join>
    </class>
</hibernate-mapping>

另外，还有一种方式就是基于外键的单向1：1关系，想一下，既然1员工对应1部门并且始终是1：1，那么为什么要每张表中用一个字段来记录主键信息呢，为什么不只在一张表中记录就好了？于是，就有了下面一种配置方式。

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="com.longtech.hibernate.domain.Employee"
        table="Employee"> 
        <id name="id" column="id" type="java.lang.Long" >
<!--             <meta attribute="field-description">唯一标识</meta>
            <generator class="native"/> -->
            <generator class="foreign">
                <param name="property">department</param>
            </generator>
        </id>
        <property name="name" type="java.lang.String" column="name" length="20">
            <meta attribute="field-description">员工名称</meta>
        </property>
        <!-- <many-to-one name="department" column="dept_id" cascade="all"></many-to-one> -->
<!--         <join table="employ_depart">
            <key column="empid"></key>
            <many-to-one name="department" cascade="all" column="deptid" unique="true"></many-to-one>
        </join> -->
        <one-to-one name="department"></one-to-one>
    </class>
</hibernate-mapping>

这种方式需要使用<one-to-one>标签，它需要指定一个name属性，作为关联时的属性名。