How to insert child element referencing foreign key in NHibernate? - fluent-nhibernate

Question

I am still new to Fluent NHibernate. Not sure how I should approach this.
I have two entities:
public class Student
{
public virtual Guid StudentId { get; set; }
public virtual String 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 Department
{
public virtual int Dept_id { get; set; }
public virtual String Dept_name { get; set; }
public virtual IList<Student> Students { get; set; }
public Department()
{
Students = new List<Student>();
}
}
and the mappings are :
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);
Map(x => x.Dept_id).Column("Dept_id");
References(x => x.Department).Column("Dept_id").Not.Nullable();
}
}
when i am trying to insert as :
StudentRepository rep = new StudentRepository();
Student s = new Student();
s.Name = txtname.Text;
s.Age = int.Parse(txtage.Text);
s.Address = txtaddress.Text;
s.Dept_id = dddept.SelectedItem.Value;
rep.Add(s);
It's giving me error as:
{"not-null property references a null or transient value nHibernateTest.Domain.Student.Department"}

Answer 1

If you will be doing this like is in your post you will get an error because you want to send empty data of Department object. As Cole W says. You need to compare id of Department which you want to add with id of existing Department from database.
I will define SelectedDepartment from Cole W answer. You can add it to repository.
//repository
public Department SelectedDepartment(int id)
{
Department getDepartment = session.Get<Department>(id);
return getDepartment;
}
//controller
s.Department = rep.SelectedDepartment(1) //for example department with id = 1

Answer 2

When you are creating your new Student you need to set the Department not just the Id. Is there a reason you need a separate property for this? I would remove this property from your entity and from your mapping class and just leave in the Department reference. If you need the Department Id you can just reference it via Student.Department.Dept_id.
Basically you are trying to insert an entity with a null value in a foreign key column. You would need to change your creation to something like this:
StudentRepository rep = new StudentRepository();
Student s = new Student();
s.Name = txtname.Text;
s.Age = int.Parse(txtage.Text);
s.Address = txtaddress.Text;
s.Department = SelectedDepartment; //SelectedDepartment is the actual department entity from the database
rep.Add(s);