title: Can nHibernate Generate Value For Property?
body: <p>Id columns can specify a generator.  Perfect.  </p>
  
  <p>However, I have a situation in which I need to assign a generated value to a property column if it is null.  Think something along the lines of an invoice table where there's a surrogate primary key column and an Invoice Number column.</p>
  
  <p><code>CREATE TABLE dbo.Invoice (InvoiceId  INT NOT NULL PRIMARY KEY IDENTITY(1,1), InvoiceNumber INT NOT NULL, InvoiceDate DATETIME NOT NULL, remaining columns...)</code></p>
  
  <p>This isn't a default value.  The Invoice Number might have certain rules around it's generation.  I have a C# class that handles the actual generation, I just need to find a way to populate the value of the property on the Invoice class.  </p>
  
  <p>Ideally I'd be able to specify a generator for a property like I do for an Id in the mapping.  Unfortunately, that doesn't seem possible from what I can determine.  I'm hoping I'm wrong.  </p>
  
  <p>I have other, non-nHibernate options I could employ like using a trigger, inserting the generation into the provider.  However, I'd like to keep it a configuration option rather than a coding option.  </p>
  
  <p>Does anyone know if a such an ability exists?</p>