title: Why unidirectional many-to-one association in hibernate is optional
body: <p>The <a href="http://docs.jboss.org/hibernate/orm/3.3/reference/en-US/html/associations.html#assoc-unidirectional-join-m21" rel="nofollow">hibernate document</a> says the <code>A unidirectional many-to-one association on a join table is common when the association is optional.</code>.</p>
  
  <p>Please let me know why it is called as <code>optional</code> here?</p>
  
  <pre><code>&lt;class name="Person"&gt;
      &lt;id name="id" column="personId"&gt;
          &lt;generator class="native"/&gt;
      &lt;/id&gt;
      &lt;join table="PersonAddress" 
          optional="true"&gt;
          &lt;key column="personId" unique="true"/&gt;
          &lt;many-to-one name="address"
              column="addressId" 
              not-null="true"/&gt;
      &lt;/join&gt;
  &lt;/class&gt;
  </code></pre>