managing lookup in MVC2 and persisting object with Nhibernate - asp.net-mvc

Question

My simplified domain model looks something like this:
public abstract class Entity<IdK>
{
public virtual IdK Code { get; protected set; }
}
public class Contact : Entity
{
public virtual string Name { get; set; }
public virtual Company Company { get; set; }
}
public class Company : Entity
{
public virtual string Name { get; set; }
}
and I've defined a viewmodel:
public ContactViewModel()
{
public Guid Code { get; set; }
public int Version { get; set; }
public string Name { get; set; }
public string Company { get; set; }
public List<SelectListItem> Companies { get; set; }
}
to manage my contacts in a view.
Since I want the user to be able to choose from a list of companies I've added a list of SelectedListItem which will be rendered in my view like this:
<%=Html.ListBoxFor(m => m.Company, (List<System.Web.Mvc.SelectListItem>)Model.Companies)%>
Now, when the user submits my form I remap my viewmodel with my model before I save it.
I populate my Contact and use the id of the ContactViewModel.Company to create an object of type Company to associate with the property of the Contact class.
Since I don't want to fetch the whole company from the database I just fill the id.
When I persist my contact, though, I get an exception: "not-null property references a null or transient Domain.Contact.Company".
What is the best solution to manage lookups and persistence with MVC + Nhibernate?
Do you have any suggestions from your experience?

Answer 1

Unfortunately with NHibernate and lookups you can't just assign the ID property to a new instance of the Company object and then assign that Company object to the Contact.
Generally what I would do is in my repository, assuming that you can't change the Company information when saving a contact is something like this:
public Contact Save(Contact contact)
{
if(contact.Company.Id > 0)
contact.Company = Session.Load<Company>(contact.Company.Id);
Session.SaveOrUpdate(contact);
}
I generally find this allows you to encapsulate the logic of loading the Company and also allows you to keep it all wrapped up nicely in a single session.
Using Session.Load in this manner avoids hitting the database as described here
If you don't do this, what you're essentially saying to NHibernate is that you have a company object which you have assigned an ID and now want to save it with all the properties set to Null or empty string values or whatever and that is not what you want.
Alternatively you could create a Save specific Domain Object that looks like this:
public abstract class Entity<IdK>
{
public virtual IdK Code { get; protected set; }
}
public class SavableContact : Entity
{
public virtual string Name { get; set; }
public virtual IdK CompanyId { get; set; }
}
Which maps directly to the Contact table in your database so that when you Save this entity you can literally just map back the CompanyId from your view model and NHibernate will only save that value back and not care at all about the company objects.
It's a case of working out what works best for you. I personally prefer the first option as the extra bit of logic helps simplify the domain model, however if you're creating and exposing a public API then the second method might make more sense.