多对一(many to one)
单向many-to-one关联是最常见的单向关联关系。

<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="addressId"
not-null="true"/>
</class>

<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key, addressId
bigint not null )
create table Address ( addressId bigint not null primary key )

一对一（one to one）
基于外键关联的单向一对一关联和单向多对一关联几乎是一样的。唯一的不同就是单
向一对一关联中的外键字段具有唯一性约束。

<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="addressId"
unique="true"
not-null="true"/>
</class>

<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key, addressId
bigint not null unique )
create table Address ( addressId bigint not null primary key )

基于主键关联的单向一对一关联通常使用一个特定的id生成器。（请注意，在这个例
子中我们掉换了关联的方向。）

<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
</class>

<class name="Address">
<id name="id" column="personId">
<generator class="foreign">
<param name="property">person</param>
</generator>
</id>
<one-to-one name="person" constrained="true"/>
</class>
create table Person ( personId bigint not null primary key )
create table Address ( personId bigint not null primary key )

一对多（one to many）
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="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses">
<key column="personId"
not-null="true"/>
<one-to-many class="Address"/>
</set>
</class>

<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table Address ( addressId bigint not null primary key, personId
bigint not null )