// The Entity

public class MyEntity : EntityBase
{
    private IList<SomeKindOfRelatedType> _relatedStuff;

    public IList<SomeKindOfRelatedType> RelatedStuff
    {
        get
        {
            if (relatedStuff == null)
            {
                relatedStuff = new List<SomeKindOfRelatedType>();
            }
            if (!IsNew)
            {
                this.Initialize(/*x, y, z*/);
            }
            return relatedStuff;
        }
    }

    private IList<SomeKindOfRelatedType> relatedStuff
    {
        get { return _relatedStuff; }
        set
        {
            _relatedStuff = value;
            OnNotifyPropertyChanged("RelatedStuff");
        }
    }
}

// An extension method defined somewhere else,
// but obviously in the same assembly as the entities
// since it's called from within the entities. :\

public static void Initialize(this EntityBase entity, /*x, y, z*/)
{
    using (var session = SessionManager.Instance.GetSession())
    {
        // new transaction
        // load data from NH
    }
}