How to implement "Last 20 something" property in your domain class?

Question

Let's say I need to implement domain model for StackOverflow.

If I am doing ORM, how can I define (and map) property for fetching "last comments" and other "last" things? It looks to me like this should be reflected in the domain model.

Sometimes I might need "all comments" though...

Answer 1

I might be influenced by my ORM tool but I see this as a query rather than a domain property. The domain contains the associations, and the repository encapsulates queries such as the one you are describing.

Answer 2

You would query your repository for the last comments by a specific user. something like

IList<Comment> recentComments = repository.GetRecentComments(user,20);

You COULD do this in the model, but depending on how many comments you expect to have I'd avoid it. The model shouldn't know how to populate itself, that is the job of the repository; however, if you're using something like NHibernate it will be there.

public class User{

 public IList<Comment> Comments { get;set;}

 public IList<Comment> GetRecentComments()
 {
   // Logic
 }


}

In that implementation you're always going to load ALL the comments to get the last 20. Not a big deal if there are only 50 comments, but if there are 5,000 comments you've got quite a bit of overhead.