二、单向关系
 1.“单向一对多关系”
 基于外键关联的单向一对多关联是一种很少见的情况，并不推荐使用
 <class name="com.jason.associations.one2many.unilateralism.one2many.Person" table="Person">
  <id name="id" column="personId">
      <generator class="native"/>
  </id>
  <set name="addresses" cascade="all">
      <!--key 是Address表的字段，最后要update Address表.personId = Person表.personId-->
      <key column="personId"  not-null="true"/>
      <one-to-many 
          class="com.jason.associations.one2many.unilateralism.one2many.Address"/>
  </set>
  <property name="name" column="personName" type="string" />
 </class>
 
 <class name="com.jason.associations.one2many.unilateralism.one2many.Address" table="Address">
     <id name="id" column="addressId">
         <generator class="native"/>
      </id>
     <property name="country" column="country" type="string" />
     <property name="city" column="city" type="string" />
 </class>
 <!--
     create table Person ( personId bigint not null primary key, personName varchar(20))
     create table Address ( addressId bigint not null primary key, personId bigint not null, country varchar(20), city varchar(20) )
 -->
 
 
 2.“单向多对一关系”
  <class name="com.jason.associations.one2many.unilateralism.many2one.Person" table="Person">
  <id name="id" column="personId">
      <generator class="native"/>
  </id>
  <many-to-one name="address" 
       class="com.jason.associations.one2many.unilateralism.many2one.Address"
       column="addressId" 
       cascade="all"
       outer-join="auto"
       update="true"
       insert="true"
       access="property"
       not-null="true"/>
  <property name="name" column="personName" type="string" />
 </class>
 
 <class name="com.jason.associations.one2many.unilateralism.many2one.Address" table="Address">
  <id name="id" column="addressId">
      <generator class="native"/>
  </id>
  <property name="country" column="country" type="string" />
  <property name="city" column="city" type="string" />
 </class>
 <!--
     create table Person ( personId bigint not null primary key, addressId bigint not null, personName varchar(20))
     create table Address ( addressId bigint not null primary key, country varchar(20), city varchar(20) )
 -->

3、“单向一对多连接表关系”
<hibernate-mapping>
 <class name="com.jason.associations.one2many.unilateralism.one2many.table.Person" table="Person">
  <id name="id" column="personId">
      <generator class="native"/>
  </id>
  <set name="addresses" table="PersonAddress" cascade="all">
      <key column="personId"/>
      <many-to-many column="addressId"
          unique="true"
          class="com.jason.associations.one2many.unilateralism.one2many.table.Address"/>
  </set>
  <property name="name" column="personName" type="string" />
 </class>
 
 <class name="com.jason.associations.one2many.unilateralism.one2many.table.Address" table="Address">
  <id name="id" column="addressId">
      <generator class="native"/>
  </id>
  <property name="country" column="country" type="string" />
  <property name="city" column="city" type="string" />
 </class>
 <!--
     create table Person ( personId bigint not null primary key, personName varchar(20))
     create table PersonAddress ( personId bigint not null, addressId bigint not null primary key)
     create table Address ( addressId bigint not null primary key, country varchar(20), city varchar(20))
 -->

 4、“单向多对一连接表关系”
 我们认为对于这种关联关系最好使用连接表。
  <class name="com.jason.associations.one2many.unilateralism.many2one.table.Person" table="Person">
  <id name="id" column="personId">
      <generator class="native"/>
  </id>
  <property name="name" column="personName" type="string" />
  <!--join 必须放在最下面-->
  <join table="PersonAddress" optional="true">
           <key column="personId" unique="true"/>
           <many-to-one name="address" 
                  class="com.jason.associations.one2many.unilateralism.many2one.table.Address"
                  column="addressId" 
                  not-null="true" 
                  cascade="all"/>
    </join> 
 </class>
 
 <class name="com.jason.associations.one2many.unilateralism.many2one.table.Address" table="Address">
  <id name="id" column="addressId">
         <generator class="native"/>
  </id>
  <property name="country" column="country" type="string" />
  <property name="city" column="city" type="string" />
 </class>
 <!--
     create table Person ( personId bigint not null primary key, personName varchar(20))
     create table PersonAddress ( personId bigint not null primary key, addressId bigint not null)
     create table Address ( addressId bigint not null primary key, country varchar(20), city varchar(20))
 -->
 
 
 二、双向关系
 双向一对多关系就是二者的组合
 为了更好地提高执行效率，需要将“多”的一端设为主控方，就是将关联关系的维护工作交给“多”的一端。