12 Apr

NHibernate 2.0 and Cascading Deletes

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The first alpha version of NHibernate 2.0 has been released some time ago. Oren has put a nice overview of the new features and the bugs that are solved for this first drop. In spite the fact that they put the word ‘alpha’ in the version name, it is actually RTM quality!

One of the great new additions is the on-delete attribute of the key element for mapping collections:

<set name="Permissions" inverse="true" cascade="save-update"> <key name="Authorization" on-delete="cascade"/> <one-to-many class="Permission"/> </set>

This particular feature solves the issue I’ve had with one-shot deletes on inverse collections (which I described here). Configuring it this way ensures that NHibernate relies on database-level ON CASCADE DELETE foreign key constraints. This implies that when an object with a collection of child objects is deleted, a single DELETE  statement is used to remove all children at once instead of individual DELETE statements for each child object (which are called orphans at this point).

There are some things that you have to take into account here. First, this option bypasses NHibernate’s optimistic locking strategy for versioned data. Second, make sure that you configure cascade="save-update" instead of cascade="all" to prevent NHibernate from performing delete cascade checks. This is important when lazy loading is enabled for the collection because it prevents that NHibernate loads one-to-many associations into memory.

I’m really excited about this new release. Kudos to the men and women who have put so much effort in building such a wonderful ORM solution!

Till next time