QueryOver by join and add conditions by Independent if

I have a QueryOver by JoinQueryOver In Nhibernate 3.1

The Person class has a association by Identity class (one-to-one) Code is a field of Person class and FirstName is a field of Identity class.

var q = SessionInstance.QueryOver<Person>()
        .Where(p => p.Code.IsLike(code,MatchMode.Start))
        .Full.JoinQueryOver(p => p.Identity);

if (!String.IsNullOrEmpty(firstName))
   q = q.Where(i => i.FirstName.IsLike(firstName, MatchMode.Anywhere));

return q.List<Person>();

that result is correct but, there is a problem. The search does not include items by null value for Code field in Person class. I corrected to following query.

var q = SessionInstance.QueryOver<开发者_运维技巧Person>()
        .Full.JoinQueryOver(p => p.Identity);

if (!String.IsNullOrEmpty(Code))
   q = q.Where(i => i.Person.Code.IsLike(code, MatchMode.Start));

if (!String.IsNullOrEmpty(firstName))
   q = q.Where(i => i.FirstName.IsLike(firstName, MatchMode.Anywhere));

return q.List<Person>();

Now i have a runtime error by this message:

could not resolve property: Identity.Code of: MyNameSpace.Domain.Entities.Identity

in a query by join between two class, How can add two condition(where) by if.

(if parameter != null)

Identity identityAlias = null;
var q = SessionInstance.QueryOver<Person>()
        .Full.JoinAlias(p => p.Identity, () => identityAlias);

if (!String.IsNullOrEmpty(code))
   q.Where(p => p.Code.IsLike(code, MatchMode.Start));

if (!String.IsNullOrEmpty(firstName))
   q.Where(() => identityAlias.FirstName.IsLike(firstName, MatchMode.Anywhere));

or

var q = SessionInstance.QueryOver<Person>();

if (!String.IsNullOrEmpty(code))
    q.Where(p => p.Code.IsLike(code, MatchMode.Start));

if (!String.IsNullOrEmpty(firstName))
    q.Full.JoinQueryOver(p => p.Identity)
        .Where(i => i.FirstName.IsLike(firstName, MatchMode.Anywhere));