NHibernate linq - Use lambda expression in place of formula attribute on mapping.

Question

NHibernate has an attribute on the property element in the mapping config named "formula" that allows the injections of sql to "calculate" a property. The issue I have is the formula using sql syntax directly. Is there a way to have nhibernate.linq to use a lambda expression instead of using the formula property.

I have the following:

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 BalanceExpression.Expression.Compile().Invoke(this); }
    }
}

public class BalanceExpression
{
    public static Expression<Func<Invoice, decimal>> Expression
    {
        get { return i => i.Amount - i.Paid; }
    }
}

<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>

I want nhibernate to use the balanceexpression.expression instead of having to put sql syntax in the formula attribute so I can remove the formula attribute from my mapping config and write queries as follows:

from i in session.linq() where i.balance > 0 select i;

How do I inject the balanceexpression.expression into the linq query?