XML : Hibernate mapping xml one-to-one (many-to-one)

i'm trying to map a relationship between two class which have one-to-one relationship. After looking up on the internet it seems like people prefer to map it using many-to-one.

for example have have a class Order and a class Bill. Bill holds a FK to the invoice.

Here are my mapping for Bill

  <?xml version="1.0"?>  <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"  "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">  <!-- Generated Mar 21, 2016 10:46:20 PM by Hibernate Tools 3.5.0.Final -->  <hibernate-mapping>      <class name="domain.Bill" table="BILL">          <id name="id" type="int">              <column name="ID" />              <generator class="native" />          </id>            <many-to-one name="order" class="domain.Order" column="ORDER_ID" unique="true" not-null="true"/>      </class>  </hibernate-mapping>    

As you can see above, in my mapping of Bill I can specified the column of Fk to the Order, but what should put in my mapping for Order since it does not have a Fk to Bill?

  <?xml version="1.0"?>  <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"  "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">  <!-- Generated Mar 21, 2016 10:46:20 PM by Hibernate Tools 3.5.0.Final -->  <hibernate-mapping>      <class name="domain.Order" table="ORDER">          <id name="id" type="int">              <column name="ID" />              <generator class="native" />          </id>            <many-to-one name="bill" class="domain.Bill" ???? fetch="select"/>        </class>  </hibernate-mapping>