NHibernate.Linq - Custom/Calculated property expression

How can a domain object include a property that calculates a value from other database mapped properties so that the calculated property can be used in both the domain object inst开发者_运维技巧ance and the db level by nhibernate.linq.

I would like to be able to use the property when working with the object directly:

Console.WriteLine(Entity.Calculated.ToString());

And when working with nhibernate.linq

var q = from e in session.Linq<Entity>()
                where e.Calculated > 0
                select e;

You need to duplicate the logic in the class and the mapping. Here's an example:

Class:

public class Invoice
{
    public virtual int Id { get; protected set; }
    public virtual decimal Amount { get; set; }
    public virtual decimal Paid { get; set; }
    public virtual decimal Balance
    {
        get { return Amount - Paid; }
    }
}

Mapping:

<class name="Invoice">
    <id name="Id">
        <generator class="hilo"/>
    </id>
    <property name="Amount"/>
    <property name="Paid"/>
    <property name="Balance" formula="Amount - Paid" access="readonly"/>
</class>

Now you can use Linq and query on Invoice.Balance