Illegal access to loading collection in Fluent and Nhibernate - fluent-nhibernate

Question

I'm not able to understand why the problem is occurring. Please let me know if there are any errors, I am very new to this topic.
public class Department
{
public virtual int Dept_id { get; set; }
public virtual String Dept_name { get; set; }
public virtual IList<Student> Students { get; set; }
//public virtual ICollection<Student> Students { get; set; }
public Department()
{
Students = new List<Student>();
}
}
public class Student
{
public Student()
{
}
//private int _Dept_id;
//public virtual Guid StudentId { get; set; }
public virtual Guid StudentId { get; set; }
/*public virtual int Dept_id
{
get { return this._Dept_id; }
set { this._Dept_id = value; }
}*/
public virtual int Dept_id { get; set; }
public virtual String Name { get; set; }
public virtual int Age { get; set; }
public virtual String Address { get; set; }
public virtual Department Department { get; set; }
}
public class DepartmentMap : ClassMap<Department>
{
public DepartmentMap()
{
Table("Department");
Id(x => x.Dept_id).Column("Dept_id");
Map(x => x.Dept_name).Column("Dept_name");
HasMany(x => x.Students).KeyColumn("Student_id").Inverse()
.Cascade.All();
}
}
public class StudentMap :ClassMap<Student>
{
public StudentMap()
{
Table("Student");
Id(x => x.StudentId).Column("Student_id").GeneratedBy.GuidComb();
Map(x => x.Name);
Map(x => x.Age);
Map(x => x.Address);
References(x => x.Department).Column("Dept_id")
.Not.Nullable().Not.LazyLoad();
}
}
Now when I am trying this code
[WebMethod(EnableSession = true)]
public List<Student> Students()
{
IList<Student> student = new List<Student>();
ISession session = NHibernateHelper.OpenSession();
student = session.Query<Student>().ToList();
return student.ToList();
}
it gives error in loading the students list inside the department as
illegal access to loading collection
What is lacking in this code and why this is happening?

Answer 1

sorry my bad !! there are cetain changes i made which made it working .. though not sure of apparent shortcomings of the mentioned idea below
changed student class as :
public class Student
{
public Student()
{
}
public virtual Guid StudentId { get; set; }
public virtual int Dept_id
{
get { return Department.Dept_id; }
set { this.Dept_id = Department.Dept_id; }
}
public virtual String Name { get; set; }
public virtual int Age { get; set; }
public virtual String Address { get; set; }
public virtual Department Department { get; set; }
}
and student mapping for the reference as
References(x => x.Department).Column("Dept_id").Cascade.All();
Note : there should be no single Dept Id mapping
and changed the DepartmentMap as :
public DepartmentMap()
{
Table("Department");
Id(x => x.Dept_id).Column("Dept_id");
Map(x => x.Dept_name).Column("Dept_name");
HasMany(x => x.Students).KeyColumn("Dept_id").AsBag();
}