Отображение NHibernate не добавляет параметр ON DELETE CASCADE к ссылке внешнего ключа

Вот мое отображение NHibernate.

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="HelloNHibernate" namespace="HelloNHibernate">
  <class name="Showing" table="showing">
    <id name="Id" column="showing_id">
      <generator class="identity"/>
    </id>
    <many-to-one class="Theater" name="Theater" column="theater_id" foreign-key="fk_showing_theater_theater_id" cascade="delete" lazy="false" fetch="join"/>
    <many-to-one class="Movie" name="Movie" column="movie_id" foreign-key="fk_showing_movie_movie_id" cascade="delete" lazy="false" fetch="join" />
  </class>
</hibernate-mapping>

Вот SQL (PostgreSQL), сгенерированный Инструмент SchemaExport:

CREATE TABLE showing
(
  showing_id serial NOT NULL,
  theater_id integer,
  movie_id integer,
  CONSTRAINT showing_pkey PRIMARY KEY (showing_id),
  CONSTRAINT fk_showing_movie_movie_id FOREIGN KEY (movie_id)
      REFERENCES movie (movie_id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION,
  CONSTRAINT fk_showing_theater_theater_id FOREIGN KEY (theater_id)
      REFERENCES theater (theater_id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION
)

Что я делаю не так? Спасибо!