使用連接表的單向關聯（Unidirectional associations with join tables）

1.one-to-many:基於連接表的單向一對多關聯，應該優先被採用。請注意，通過制定unique=“true”，我們可以把多樣性從多對多改變爲一對多。

<class name="Person">
	<id name="id" column="personId">
		<generator class="native"/>
	</id>
	<set name="addresses" table="PersonAddress">
		<key column="personId"/>
		<many-to-many column="addressId"
			unique="true"
			class="Address"/>
	</set>
</class>

<class name="Address">
	<id name="id" column="addressId">
		<generator class="native"/>
	</id>
</class>

2.many-to-one:基於連接表的單向多對一關聯在關聯關係可選的情況下應用也很普遍。

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

<class name="Address">
	<id name="id" column="addressId">
		<generator class="native"/>
	</id>
</class>

3.one-to-one:基於連接表的單向一對一關聯也是可行的，但非常少見。

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

<class name="Address">
	<id name="id" column="addressId">
		<generator class="native"/>
	</id>
</class>

4.many-to-many：多對多

<class name="Person">
	<id name="id" column="personId">
		<generator class="native"/>
	</id>
	<set name="addresses" table="PersonAddress">
		<key column="personId"/>
		<many-to-many column="addressId"
			class="Address"/>
	</set>
</class>

<class name="Address">
	<id name="id" column="addressId">
		<generator class="native"/>
	</id>
</class>