NHibernate SchemaExport with generated properties

<id name="Id" column="Id" type="Guid">
  <generator class="guid.comb" />
</id>
<property name="OrderNumber" column="OrderNumber" type="Int32" generated="insert"/>

<property name="OrderNumber" generated="insert" update="false">
  <column name ="OrderNumber" sql-type="int identity" not-null="true"/>
</property>