NHibernate: inserting additional calculated column

Question

What would be your approach with NHibernate to do the following:
When Comment is inserted into DB, and its property Text is inserted into column Text (due to a mapping), insert into column TextHash a hash of this property value.

It seems trivial if I map TextHash, but how should I do this without mapping it?
I do not need it in the domain model, its a DB-level optimization (I would use computed HashBytes if not for the length limit).

Answer 1

Well, if you don't want to use the property, declare it private and calculate it when you set the text property. It's not exactly "without mapping it", but you won't see it when you're using the class (disclaimer: advice based on Hibernate, as opposed to NHibernate).

Other than that, you could always write a trigger, although I'd personally much rather add a private field and be done with it.

Answer 2

Here's a suggestion: As far as I know you can't access a column in your DB using NHibernate if it there no mapping defined for it. To prevent other parts of your application to see this field of your class you can define its access as field in your mapping so that it can be private an no one knows that it exists but NHibernate: In your class:

private string _textHash

In your mapping:

<property name='_textHash' column='TextHash' access='field' />

Answer 3

There are some similar questions, such as this one:
Unmapped Columns in NHibernate?

However, IInterceptor seems like an overkill for a change in a single entity insert. And EventListeners are less than perfectly documented and also somewhat too complex for a single column.

So I have decided on the solution that I see both as most reusable and most local:

  <property name="Text" type="StringWithMD5HashUserType">
    <column name="Text" length="20000" not-null="true" />
    <column name="TextHash" length="32" not-null="true" />
  </property>

Where StringWithMD5HashUserType is ICompositeUserType that reads Text from first column, but writes both Text and its Hash (I do not add the code of StringWithMD5HashUserType because it is way too long, but essentially very simple).