How to make hibernate unidirectional many-to-many association updateable?

Question

I have two entities - Category and Attribute . Category can have multiple related attributes, and Attribute can be related to any number of categories. Association should be available only on Category side - Attribute objects are not aware of categories they are related to.

So I model this association as unidirectional many-to-many:

Category.hbm.xml

<class name="Category" table="category" proxy="ICategory" entity-name="category">
  <id name="id" column="id" unsaved-value="null"><generator class="identity" /></id>
  ...some properties...
  <bag name="relatedAttributes" table="category_attribute" fetch="select">
    <key column="id_category" />
    <many-to-many column="id_attribute" entity-name="attribute" />
  </bag>
</class>

and Attribute.hbm.xml

<class name="Attribute" table="attribute" proxy="IAttribute" entity-name="attribute">
  <id name="id" column="id" unsaved-value="null" ><generator class="identity" /></id>
  ...some properties...
</class>

Mapping works perfectly with current data until it needs an update. I just want to do things as simple as these:

ICategory c = (ICategory) session.get("category", 1);
c.getRelatedAttributes().add((IAttribute) session.get("attribute", 2));
session.update("category", c);

How can i make this association updateable?

Answer 1

Finally done. Changes that affect behavior:

<bag name="relatedAttributes" table="category_attribute" fetch="select" inverse="false" cascade="save-update">
  ...
</bag>

and do not forget to call session.flush() after operations.