Question

Suppose I have

public class Product: Entity
{
   public IList<Item> Items { get; set; }
}

Suppose I want to find an item with max something… I can add the method Product.GetMaxItemSmth() and do it with Linq (from i in Items select i.smth).Max()) or with a manual loop or whatever. Now, the problem is that this will load the full collection into memory.

The correct solution will be to do a specific DB query, but domain entities do not have access to repositories, right? So either I do

productRepository.GetMaxItemSmth(product)

(which is ugly, no?), or even if entities have access to repositories, I use IProductRepository from entity

product.GetMaxItemSmth() { return Service.GetRepository<IProductRepository>().GetMaxItemSmth(); }

which is also ugly and is a duplication of code. I can even go fancy and do an extension

public static IList<Item> GetMaxItemSmth(this Product product)
{
   return Service.GetRepository<IProductRepository>().GetMaxItemSmth();
}

which is better only because it doesn’t really clutter the entity with repository… but still does method duplication.

Now, this is the problem of whether to use product.GetMaxItemSmth() or productRepository.GetMaxItemSmth(product)… again. Did I miss something in DDD? What is the correct way here? Just use productRepository.GetMaxItemSmth(product)? Is this what everyone uses and are happy with?

I just don’t feel it is right… if I can’t access a product’s Items from the product itself, why do I need this collection in Product at all??? And then, can Product do anything useful if it can’t use specific queries and access its collections without performance hits?

Of course, I can use a less efficient way and never mind, and when it’s slow I’ll inject repository calls into entities as an optimization… but even this doesn’t sound right, does it?

One thing to mention, maybe it’s not quite DDD… but I need IList in Product in order to get my DB schema generated with Fluent NHibernate. Feel free to answer in pure DDD context, though.

UPDATE: a very interesting option is described here: http://devlicio.us/blogs/billy_mccafferty/archive/2007/12/03/custom-collections-with-nhibernate-part-i-the-basics.aspx, not only to deal with DB-related collection queries, but also can help with collection access control.

Answer 1

Having an Items collection and having GetXXX() methods are both correct.

To be pure, your Entities shouldn’t have direct access to Repositories. However, they can have an indirect reference via a Query Specification. Check out page 229 of Eric Evans’ book. Something like this:

public class Product
{
    public IList<Item> Items {get;}

    public int GetMaxItemSmth()
    {
        return new ProductItemQuerySpecifications().GetMaxSomething(this);
    }
}

public class ProductItemQuerySpecifications()
{
   public int GetMaxSomething(product)
   {
      var respository = MyContainer.Resolve<IProductRespository>();
      return respository.GetMaxSomething(product);
   }
}

How you get a reference to the Repository is your choice (DI, Service Locator, etc). Whilst this removes the direct reference between Entity and Respository, it doesn’t reduce the LoC.

Generally, I’d only introduce it early if I knew that the number of GetXXX() methods will cause problems in the future. Otherwise, I’d leave it for a future refactoring exercise.