1. 1) Download the NHibernate source and go to the “src\NHibernate“ directory
2. 2) Copy nhibernate-configuration-2.0.xsd, nhibernate-mapping-2.0.xsd, and nhibernate-generic.xsd to the Visual Studio “schemas“ directory.  By default, the schemas directory is “C:\Program Files\Microsoft Visual Studio 8\Xml\Schemas“.
3. 3) Restart Visual Studio

It's a calculated field. SQL conditions are directly evaluated by the database. For example, I saw a case where it was used a "Case When" of SQL.

property name="Total" formula="Subtotal - DiscountAmount" type="Nullables.NHibernate.NullableDecimalType, Nullables.NHibernate" />

In this case, two fields referred to class are subtracted from the table.

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