Bidirectional one to many (or many to one) cascade delete behaviour. It works, but why? - nhibernate

Question

I have two Nhibernate mappings for two classes, Category and Product. My Category class has two properties that are collections. The Children property is a collection of type Category which represents child categories (represents a category menu, typical parent child scenario). The second property on the Category class is a Products collection which represents all the products under a category.
What I am trying achieve is when I delete a category I want the category to deleted but not the product. So I want the product to be orphaned. i.e have its foreign key (CategoryId) in the Product table set to null. I don't want to delete a product just because I have deleted a category. I want to be able to reassign in at a later time to another category. My mappings representing the mentioned scenario are below.
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="naakud.domain" namespace="naakud.domain">
<class name="Category">
<id name="Id">
<generator class="hilo" />
</id>
<version name="Version"/>
<property name="Name" not-null="true" unique="true" />
<set name="Products"
cascade="save-update"
inverse="true"
access="field.camelcase-underscore">
<key column="CategoryId" foreign-key="fk_Category_Product" />
<one-to-many class="Product" />
</set>
<many-to-one name="Parent" class="Category" column="ParentId" />
<set name="Children"
collection-type="naakud.domain.Mappings.Collections.TreeCategoriesCollectionType, naakud.domain"
cascade="all-delete-orphan"
inverse="true"
access="field.camelcase-underscore">
<key column="ParentId" foreign-key="fk_Category_ParentCategory" />
<one-to-many class="Category"/>
</set>
</class>
</hibernate-mapping>
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="naakud.domain" namespace="naakud.domain">
<class name="Product">
<id name="Id">
<generator class="hilo" />
</id>
<version name="Version" />
<property name="Name" not-null="true" unique="true" />
<property name="Description" not-null="true" />
<property name="UnitPrice" not-null="true" type="Currency" />
<many-to-one name="Category" column="CategoryId" />
</class>
</hibernate-mapping>
With this mapping, when I delete a category which has products associated with it I get the following constraint error.
The DELETE statement conflicted with the REFERENCE constraint "fk_Category_Product". The conflict occurred in database "naakud", table "dbo.Product", column 'CategoryId'.
The statement has been terminated.
However, when I remove the inverse=true attribute on the Products collection in the Category mapping then it works fine. My CategoryId foreign key in the products table is set to null and thus disassociating a product with a category. Which is what I want.
I have read about the inverse attribute and I understand it signifies the owning side of a relationship and updates/inserts/deletes are done in a different order which is why I think it solves my problem. So my question is, am I solving my problem in the correct way? How does this affect performance? (not much I suspect). Would it be better to have a uni-directional relationship without the many to one side and have the inverse attribute set to true to get better performance? Or am I going crazy and completely missing the point?

Answer 1

Another way of fixing the delete problem is by setting the many-to-one property to null on all the related entities to null before flushing.
I can think of at least two ways to do it:
In the same method that calls session.Delete(category), do:
foreach (var product in category.Products)
product.Category = null;
Using HQL:
session.CreateQuery(
"update Product set Category = null where Category = :category")
.SetParameter("category", category)
.ExecuteUpdate();
Update:
Here's a proof-of-concept implementation using an event listener.

Answer 2

I assume that you read about Inverse Attribute in NHibernate
As the error message says, your DELETE generates a conflict with the foreign key constraint, meaning that the DB cannot delete the Category as long as there are Products referencing that particular Category.
What you could do (if you can alter the DB schema) is applying "ON DELETE SET NULL" to your foreign key constraint. That way, when the DELETE is executed, the DB will automatically set all references in the Product table to NULL.
If you cannot modify the foreign key, then you would have little choice but to remove the inverse attribute. Doing so will result in NHibernate first setting the Product.Category reference to NULL and then deleting the Category.
If you need Product.Category fairly often then you should not get rid of the many-to-one attribute in Product.
Regarding the performance, that depends on how often you insert Products. Each insert will result in an additional update to set the foreign key. That should not be a problem, though.