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?

Answer 1

Since you already have the Amount and Paid properties on your object, you can just define the Balance property as a regular property:

public int Balance { get { return Amount - Paid; } }

If you don’t set the Amount / Paid properties to ‘Lazy’, I think this is the best, most readable and most maintainable solution.
If either of these properties IS lazy-loaded, you can still use this method, just note that it would have side effects.
For example- a statement such as this

invoices.Where(i => i.Balance > 5)

would cause a DB access for each Invoice in the collection.
This might still be acceptable for you, just be aware of that..
Cheers
Jhonny