Entity Framework testing with IRepository - problem with lazy loading

Question

I am refactoring an MVC project to make it testable. Currently the Controller uses the Entity Framework's context objects directly to ask for the required data. I started abstract this and it just doesn't work. Eventually I have an IService and an IRepository abstraction, but to describe the problem let's just look at the IRepository. Many people advise an interface with functions which return some of these: IQueriable<...>, IEnumerable<...>, IList<...>, SomeEntityObject, SomeDTO. Then when one wants to test the service layer they can implement the interface with a class which doesn't go to the database to return these.

Problem: Using linq to entities I have lazy (deferred) loading in my toolset. This is actually very useful, because my controller action functions know which data they need for the view and I didn't ask for more than required. However linq to anythingelse doesn't have lazy loading. So when my IRepository functions return any of the above mentioned things I lose lazy loading. I extended my interface with functions like "GetAnything" and "GetAnythingDeep" but it's not enough: it has to be much more fine-grained. Which would result about 5-6 functions for the same type of object, depending on the properties I want to get in the result. Maybe could be a general function with some "include properties" parameter, but I don't like that too.

Eventually atm I think if I want to make it testable that will result either much less efficient or much more complicated code. Sounds not right.

Btw I was thinking about to change the data source behind the entity model to either xml or some object data soruce, and so I could keep the linq to entities. I found that it's not supported out of the box... which is also sad: this means that entity framework means database source - not a really useful abstraction.

Specific example:

Entity objects: Article, Language, Person. Relations: Article can have 1-N languages, and one Person (publisher).

ViewModel object: ArticleDeepViewModel: Contains all the properties of the article, including the languages and the Name of the Person (it's for view the article, so no need for the other properties of the person).

Controller action which will return this view should get the data from somewhere.

Code before modifications:

    using (var context = new Entities.Articles())
        {
            var article = (from a in context.Articles.Include("Languages")
                       where a.ID == ID
                       select new ViewArticleViewModel()
                       {
                       ID = a.ID,
                       Headline = a.Headline,
                       Summary = a.Summary,
                       Body = a.Body,
                       CreatedBy = a.CreatedByEntity.Name,
                       CreatedDate = a.CreatedDate,
                       Languages = (from l in context.Languages select new ViewLanguagesViewModel() { ID = l.ID, Name = l.Name, Selected = a.Languages.Contains(l) })}).Single();
        this.ViewData.Model = article;
    }
    return View();

Code after modifications could be something like:

var article = ArticleService.GetArticleDeep(ID);
var viewModel = /* mapping */
this.ViewData.Model = viewModel;
return View();

Problem is that GetArticleDeep should return an Article object with Languages included and the entire Person object included (it shouldn't know that the viewmodel needs just the Name of the Person). Also I have so far 3 different viewmodels for an article. For example if someone wants to see the list of articles, then it's unnecessary to get the languages, the body and some other properties, however it might be useful to get the Name of the publisher (which is in the deep). Before "testable" code the controller actions could just contain the linq to entities query and get whichever data they need using lazy loading, Include function, using subqueries, referencing foreign properties (Publisher.Name) ... So there is no unnecessary query to the database and no unnecessary data transferred from the database.

What should be the IService or IRepository interface provide to get the 3-4 different level of Article objects or sometimes list of these objects?

Answer 1

Not sure if you are planning to stick with lazy loading, but if you want a flexible way to integrate eager loading into your repository and service layers first check out this article:

http://blogs.msdn.com/b/alexj/archive/2009/07/25/tip-28-how-to-implement-include-strategies.aspx

He basically gives you a way to build a strongly-typed include strategy like this:

var strategy = new IncludeStrategy<Article>();
strategy.Include(a => a.Author);

Which can then be passed into a general method on your repository or service layers. This way you don't have to have a separate method for each circumstance (i.e. your GetArticleDeep method).

Here is an example repository method using the above include strategy:

public IQueryable<Article> Find(Expression<Func<Article, bool>> criteria, IncludeStrategy<Article> includes)
{
    var query = includes.ApplyTo(context.Articles).Where(criteria);
    return query;
}