class hierarchy - NHibernate - Changing sub-types

Question

How do you go about changing the subtype of a row in NHibernate? For example if I have a Customer entity and a subclass of TierOneCustomer, I have a case where I need to change a Customer to a TierOneCustomer but the TierOneCustomer should have the same Id (PK) as the original Customer entity.

The mapping looks something like this:

<class name="Customer" table="SiteCustomer" discriminator-value="C">
  <id name="Id" column="Id" type="Int64">
    <generator class="identity" />
  </id>
  <discriminator column="CustomerType" />
  ... properties snipped ...

  <subclass name="TierOneCustomer" discriminator-value="P">
    ... more properties ...
  </subclass>
</class>

I'm using the one-table per class hierarchy model, so using plain-sql, it'd be just a matter of a sql update of the discriminator (CustomerType) and set the appropriate columns relevant for the type. I can't find the solution in NHibernate, so would appreciate any pointers.

I'm also thinking whether the model is correct considering this use-case, but before I go down that route, I want to make sure doing as described above is actually possible in the first place. If not, I'll almost certainly think about changing the model.

Answer 1

Short answer is yes, you can change the discriminator value for the particular row(s) using native SQL.

However, I don't think NHibernate is intended to work this way, since the discriminator is generally "invisible" to the Java layer, where its value is supposed to be set initially according to the class of the persisted object and never changed.

I recommend looking into a cleaner approach. From the standpoint of the object model, you're trying to convert a superclass object into one of its subclass types while not changing the identity of its persisted instance, and that's where the conflict is (the converted object isn't really supposed to be the same thing). Two alternative approaches are:

• Create a new instance of TierOneCustomer based on the information in the original Customer object, then delete the original object. If you were relying on the Customer's Primary Key for retrieval, you'll need to take note of the new PK.

or

• Change your approach so the object type (discriminator) doesn't need to change. Instead of relying on a subclass to distinguish TierOneCustomer from Customer, you can use a property that you can modify freely at any time, i.e. Customer.Tier = 1.

Here are some related discussions on the Hibernate Forums that may be of interest:

1. Can we update the discriminator column in Hibernate
2. Table-per-Class Problem: Discriminator and Property
3. Converting a persisted instance into a subclass