IQueryable and Many-to-Many relationship

Question

Hi folks,

I have a situation where i have several Categories, containing Articles. However, it is not just a simple Category-contains-Article relationship, but rather each Article has one Category set as its Main and several other Categorys set as Secondarys, ie:

public class Category
{
   ...
   public LazyList<Article> Articles { get; set; }
   ...
}

public class Article
{
   ...
   public Category MainCategory { get; set; }
   public LazyList<Category> Categorys { get; set; }
   ...
}

I am populating in a repository with:

public IQueryable<Category> GetCategorys()
{
   return from c in db.WOTS_Categories 
      let articles = GetArticles(c.Id, true)
      orderby c.Name
      select new Category(c.Id, c.Name)
      {
         ...
         Articles = new LazyList<Article>(articles),
         ...
      };
}

and...

public IQueryable<Article> GetArticles()
{
   return from a in db.WOTS_Articles
   let main = GetCategorysForArticle(a.Id, true).SingleOrDefault()
   let cats = GetCategorysForArticle(a.Id, false)
   orderby a.CreatedOn descending
   select new Article
   {
      ...
      MainCategory = main,
      Categorys = new LazyList<Category>(cats),
      ...
   };
}

and...

public IQueryable<Category> GetCategorysForArticle(Guid articleId, bool isMain)
{
   return from c in GetCategorys()
   join ce in db.WOTS_CategoryEntries on c.Id equals ce.CategoryId
   where ce.ArticleId == articleId && ce.IsMain == isMain
   orderby c.Name
   select c;
}

The problem i am having though, is it errors with "GetCategorysForArticle(System.Guid, Boolean)' has no supported translation to SQL"

Can anyone assist in diagnosing the problem, or suggesting a better way of achieving what i want?