title: Make Projection on discriminator-value
body: <p>I have this NHibernate mapping</p>
  
  <pre><code>&lt;hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"&gt;
    &lt;class name="ParentClass" table="myclass" abstract="true" &gt;
      ...
      &lt;discriminator column="mytype" type="string" /&gt;
      ...
      &lt;subclass name="SubClass1" discriminator-value="Type1" /&gt;
      &lt;subclass name="SubClass2" discriminator-value="Type2" /&gt;
    &lt;/class&gt;
  &lt;/hibernate-mapping&gt;
  </code></pre>
  
  <p>So how can I get Projection of discriminator-value like any property in my DAO when I filtering ParentClass objects?
  Code in DAO:</p>
  
  <pre><code>...
    using (ISession session = NHibernateSession)
    {
        ICriteria criteria = session.CreateCriteria&lt;ParentClass&gt;("p")
        ...
        criteria.SetProjection(Projections.ProjectionList()
            .Projections.Property("p.Property1")
            .Projections.???  // something to get discriminator-value from myclass.mytype column
        ...
    }
  ...
  </code></pre>