nHibernate QueryOver with a Left Join using *OR* and *AND* clauses

After some hours looking for the answer I failed. Maybe is too simple and no one had never talked about it, maybe I didn't used the right terms to search.

Basically I need to use OR and AND clause on a LEFT JOIN table. And I wish to do that in a QueryOver. The resulting SQL should look like this:

SELECT customer.name, order.id
  FROM customer
  LEFT JOIN order ON (order.idcustomer = customer.id)
 WHERE customer.active
   AND (order.id IS NULL or (order.date >= :p0 AND order.date <= :p1)

Which means, all active customers and their orders, BUT only those who never made an order or those who have orders between a specific period.

The most close to this I got in QueryOver was:

Order order = null;

var query = session.QueryOver<Customer>()
   .Left.JoinAlias(x => x.Orders, () => order)
   .Where(x => x.Active)
   .And(() => order == null || (order.date >= date1 && order.date <= date2))

The above QueryOver, before even is executed, gives me 'Reference not set'. I found the instructions about using 'Restrictions', but none (or at least, none clear enough to my limited mind) using OR and AND together.

I appreciate any help.

I think you're just missing a .Id on order in your where clause:

Order order = null;

var query = session.QueryOver<Customer>()
   .Left.JoinAlias(x => x.Orders, () => order)
   .Where(x => x.Active)
   .And(() => order.Id == null || (order.date >= date1 && order.date <= date2))
// -----------------^

This gives me the following SQL (SQL Server):

SELECT
    this_.Name as y0_,
    order1_.Id as y1_ 
FROM
    Customer this_ 
    LEFT OUTER JOIN [Order] order1_ ON this_.Id = order1_.CustomerId
WHERE
    this_.Active = @p0 AND (
        order1_.Id IS NULL OR (
            order1_.Date >= @p1 AND order1_.Date <= @p2
        )
    );