# 7.4.2\. 一对一（one to one） _基于外键关联的双向一对一关联_也很常见。 ``` <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> <one-to-one name="person" property-ref="address"/> </class> ``` ``` create table Person ( personId bigint not null primary key, addressId bigint not null unique ) create table Address ( addressId bigint not null primary key ) ``` _基于主键关联的一对一关联_需要使用特定的id生成器。 ``` <class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <one-to-one name="address"/> </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 ) ```