LINQ to Entities ToString() - None of the proposed solutions work?

Question

I'm posting this more because I'd like to learn more, because my work-around has been to basically avoid using LINQ to Entities! It would nice if I could use it though. So if anyone can enlighten me..

I'm working on an ASP.Net MVC 3 application (my first one) using code-first Entity Framework as my data model. It's going to be an online shop and I am working on the 'Admin' controller where, as an admin, you will be able to edit products etc. When it comes to adding a new product, you must specify a category, which means a dropdown list of categories.

I started off by just returning an IEnumerable collection of categories in the ViewBag, based on the information I found here. This seemed like a really nice way to approach things.

But I got the same error you'll see below. So following a suggestion I read online, I created a separate model specifically for creating new and editing existing products. The Categories collection is used to build a dropdown list in the view.

public class ProductModel
{
    public int Id { get; set; }
    public int CategoryId { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public double Price { get; set; }
    public bool IsVisible { get; set; }

    public IEnumerable<SelectListItem> Categories
    {
        get
        {
            return new DataAccess()
                .Categories
                .OrderBy(c => c.Description)
                .Select(c => new SelectListItem
                    {
                        Value = c.Id.ToString(),
                        Text = c.Description
                    });
        }
    }
}

This results in the following error message: "LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression.". One proposed online solution was to use the SqlFunction methods provided by the Entity Framework. So I tried this instead ...

    public IEnumerable<SelectListItem> Categories
    {
        get
        {
            return new DataAccess()
                .Categories
                .OrderBy(c => c.Description)
                .Select(c => new SelectListItem
                    {
                        Value = SqlFunctions.StringConvert((double)c.Id),
                        Text = c.Description
                    });
        }
    }

But this results in this error message: "The specified method 'System.String StringConvert(System.Nullable`1[System.Double])' on the type 'System.Data.Objects.SqlClient.SqlFunctions' cannot be translated into a LINQ to Entities store expression.".

I've even tried adding a read-only property to the data class called IdAsString, but this obviously makes the Entity Framework very unhappy!

In the end, I realised that it wasn't worth all the time I was putting into it, and just stopped using LINQ altogether. This works fine, but it would be nice to know if there is a "correct" way to do this that actually works!

This is my current working solution:

    public IEnumerable<SelectListItem> Categories
    {
        get
        {
            var dal = new DataAccess();
            var categories = dal.Categories;
            var items = new List<SelectListItem>();

            foreach (var category in categories)
                items.Add(new SelectListItem
                    {
                        Value = category.Id.ToString(),
                        Text = category.Description
                    });

            return items;
        }
    }

So if anyone is already doing this and they think they're doing it the "correct" way, then I'd love to know what you think!

Answer 1

Materialize the query before you invoke ToString will make the last part a non linq-to-entities query. You could optimize this by only selecting the id and description, then calling AsEnumerable, and then getting the selectlistitems.

    return new DataAccess() 
        .Categories 
        .OrderBy(c => c.Description)
        .AsEnumerable()
        .Select(c => new SelectListItem 
            { 
                Value = c.Id.ToString(), 
                Text = c.Description 
            });