bidirectional relationship patterns with nhibernate

Question

In my domain, an Employee and a Department have a one-to-many bidirectional relationship; to let the child Employee synchronize this, I have an 'internal' access field for the Set (Iesi for NHibernate) of Employees that are in the Department, which would otherwise be readonly public. Like so:

Department class:

protected internal ISet<Employee> _staff;  
public virtual ReadOnlyCollection<Employee> Staff {  
   get { return new List<Employee>(_staff).AsReadOnly(); }   
}  
public virtual void AddStaff(Employee emp) {
    emp.Department = this;  } 
}

Employee class:

private Department _department;
public virtual Department Department {
   set {
       // check valid value, etc.
       value._staff.Add(this);
   }
}

I made access in my (FNH) mapping AsField(Prefix.Underscore), but since I can't make the Department._staff field virtual NH is not happy. I guess I could make the field a virtual property and force feed it, but that feels like I'm making the domain class overly aware of persistence.

I'm learning both NH and FNH and I know I need a good primer in relationship mapping, but my primary question for this post is the logic in my domain classes:
1) Is this a good c# programming pattern for a readonly set in a biderectional relationship?
2) What is the best way to make this more usable to NHibernate?

Thanks for sharing!
Berryl

Jamie Ide Jamie Ide · Accepted Answer · 2009-04-15T02:08:41

I implement one-to-many relationships using this pattern:

public class Department
{
    private IList<Employee> _staff = new List<Staff>();

    public virtual IEnumerable<Employee> Staff
    {
        get { return _staff; }
    }

    // Add and Remove have additional checks in them as needed

    public virtual void AddStaff(Employee staff)
    {
        staff.Department = this;
        _staff.Add(staff);
    }

    public virtual void RemoveStaff(Employee staff)
    {
        staff.Department = null;
        _staff.Remove(staff);
    }
}

public class Employee
{
    public virtual Department Department { get; internal set; }
}

In this case, Department is the inverse side of the relationship.

bidirectional relationship patterns with nhibernate

1 Answers