NHibernate mapping to another object NOT on the ID

<many-to-one name="_Township" column="ZipCode" foreign-key="ZipCode" class="HQData.Objects.Township, HQData" />

<set name="_Vendors" cascade="all">
      <key  column="ZipCode" foreign-key="ZipCode"/>
      <one-to-many
         class="HQData.Objects.Vendor, HQData"/>
    </set>