R – NHibernate and lazy loading: how to get only few rows from the collection

Given your request, you could simply not map from Department to Employees, nor have an Employees property on your department. This would mean you always have to make a database hit to find the employees of a database.

Aplogies if these code examples don't work out of the box, I'm not near a compiler at the moment

So, your department class might look like:

 public class Department 
 { 
     public int Id { get; protected set; }
     public string Name { get; set; }
     /* Equality and GetHashCode here */
 }

and your Employee would look like:

 public class Employee
 { 
     public int Id { get; protected set; }
     public Name Name { get; set; }
     public Department Department { get; set; }
     /* Equality and GetHashCode here */
 }

Any time you wanted to find Employees for a department, you've have to call:

/*...*/
session.CreateCriteria(typeof(Employee))
    .Add(Restrictions.Eq("Department", department)
    .List<Employee>();

Simply because your spec says "Departments have many Employees", doesn't mean you have to map it as a bi-directional association. If you can keep your associated uni-directional, you can really get your data-access to fly too.

Google "Domain Driven Design" Aggregate, or see Page 125 of Eric Evan's book on Domain Driven Design for more information

Chris' suggestion is how I'd do it, however if you want to do it at runtime you can set the Fetchmode on your criteria to be lazy like so:

criteria.SetFetchMode("Orders", FetchMode.Lazy)