QueryOver<A>().Where(a => a.B.Count() > 0) does not work - nhibernate

Question

I get an exception: Unrecognised method call in epression a.B.Count() when I run:
var query = session.QueryOver<A>()
.Where(a => a.B.Count() > 0)
.List();
The following code works:
var query1 = session.QueryOver<A>().List();
var query2 = query1.Where(a => a.B.Count() > 0);
Any ideas? Thanks.
Edit:
Here is my mappings. I'm using NHibernate 3.1.0.4000:
Models:
public class A
{
public virtual int Id { get; private set; }
public virtual ICollection<B> Bs { get; set; }
}
public class B
{
public virtual int Id { get; private set; }
}
Mappings:
public class AMappings : ClassMap<A>
{
public AMappings()
{
Id(x => x.Id);
HasMany(x => x.Bs).LazyLoad();
}
}
public class BMappings : ClassMap<B>
{
public BMappings()
{
Id(x => x.Id);
}
}
Rest of my code:
class Program
{
static void Main(string[] args)
{
// Create connection string
string connectionString = new System.Data.SqlClient.SqlConnectionStringBuilder()
{
DataSource = #".\r2",
InitialCatalog = "TestNHibernateMappings",
IntegratedSecurity = true
}.ConnectionString;
// Create SessionFactory
ISessionFactory sessionFactory = Fluently.Configure()
.Database(MsSqlConfiguration
.MsSql2008.ConnectionString(connectionString)
.ShowSql())
.Mappings(m => m.FluentMappings
.Add(typeof(AMappings))
.Add(typeof(BMappings)))
.ExposeConfiguration(BuildSchema)
.BuildConfiguration()
.BuildSessionFactory();
// Test
var session = sessionFactory.OpenSession();
// This line works OK
var query1 = session.Query<A>()
.Where(a => a.Bs.Count() > 0);
// This line throws exception: Unrecognised method call in epression a.Bs.Count()
var query2 = session.QueryOver<A>()
.Where(a => a.Bs.Count() > 0);
}
static void BuildSchema(Configuration cfg)
{
new SchemaExport(cfg).Create(false, true);
}
}

Answer 1

QueryOver is not LINQ.
Your second code snippet works because it's retrieving ALL THE RECORDS and using LINQ-to-objects in memory.
What you should do is:
session.Query<A>()
.Where(a => a.B.Count() > 0)
.ToList();
or better yet:
session.Query<A>()
.Where(a => a.B.Any())
.ToList();
Query is an extension method, you need to add using NHibernate.Linq;