Question

Using this answer I am now able to store my projection logic in an Expression and use it inside another projections.

However, when I started to implement this approch in my solution, I found out, that I am not able to use the stored Expression on a Navigation property which is a single FK (not a collection).

The following code demonstrates this issue:

namespace Entities
{
    public class BlogPost
    {
        public virtual int BlogPostId { get; set; }
        public virtual string Title { get; set; }

        public virtual string NotUsed { get; set; }

        public virtual User Author { get; set; }
    }

    public class User
    {
        public virtual int UserId { get; set; }
        public virtual string Name { get; set; }
        public virtual string NotUsed { get; set; }

        public virtual ICollection<BlogPost> BlogPosts { get; set; }
    }
}

namespace Models
{
    public class BlogPostModel
    {
        public string Title { get; set; }
        public UserModel Author { get; set; }
    }

    public class UserModel
    {
        public string Name { get; set; }
    }

    public static class BlogPostModelExtensions
    {
        public static readonly Expression<Func<BlogPost, BlogPostModel>> ToModelConverterExpression =
            p =>
            new BlogPostModel
            {
                Title = p.Title,
                Author = null, //Problem!
                // I need to convert User (p.Author) to UserModel using UserModelExtensions.ToModelConverterExpression
            };
    }

    public static class UserModelExtensions
    {
        public static readonly Expression<Func<User, UserModel>> ToModelConverterExpression = 
            u => new UserModel{ Name = u.Name, };
    }
}

Is it possible to convert single FK navigation property to a model using Expression?

Answer 1

This is currently possible in an overly complicated manner:

p =>
new BlogPostModel
{
    ...,
    Author = new[] { p }.AsQueryable().Select(UserModelExtensions.ToModelConverterExpression).FirstOrDefault()
}

However, the generated SQL, while correct, is needlessly complicated and slow. As far as I know, there is not yet any way to directly get what you want, but I’ve been looking for the same thing, and I have a proof-of-concept patch for the open source to-be-EF-6.0 that I plan to submit for inclusion, see the discussion thread and the change.