How to reuse projections in Entity Framework? - asp.net-mvc

Question

I have an ASP.NET MVC application which uses Entity Framework to get data.
I need to transform Entites to Models before passing them to View. Projections can be very complex, but to keep it simple:
public static IQueryable<UserModel> ToModel(this IQueryable<User> users)
{
return from user in users
select new UserModel
{
Name = user.Name,
Email = user.Email,
};
}
This can be used in a controller like this:
return View(Repository.Users.ToModel().ToList());
Very good. But what if I want to use this projection inside another one? Example:
public static IQueryable<BlogPostModel> ToModel(this IQueryable<BlogPost> blogs)
{
return from blogs in blogs
select new BlogPostModel
{
Title = blog.Title,
Authors = blog.Authors.AsQueryable().ToModel(), // (entities are POCOs)
// This does not work, because EF does not understand method ToModel().
};
}
(let's suppose blog can have more then one author and it is of type User).
Can I somehow separate the projections and reuse them inside another ones?

Answer 1

Here's something that actually works (in a simple test application) to only select the requested fields:
namespace Entities
{
public class BlogPost
{
public virtual int Id { get; set; }
public virtual string Title { get; set; }
public virtual DateTime Created { get; set; }
public virtual ICollection<User> Authors { get; set; }
}
public class User
{
public virtual int Id { get; set; }
public virtual string Name { get; set; }
public virtual string Email { get; set; }
public virtual byte[] Password { get; set; }
public virtual ICollection<BlogPost> BlogPosts { get; set; }
}
}
namespace Models
{
public class BlogPostModel
{
public string Title { get; set; }
public IEnumerable<UserModel> Authors { get; set; }
}
public class UserModel
{
public string Name { get; set; }
public string Email { get; set; }
}
public static class BlogPostModelExtensions
{
public static readonly Expression<Func<BlogPost, BlogPostModel>> ToModelConverterExpression =
p =>
new BlogPostModel
{
Title = p.Title,
Authors = p.Authors.AsQueryable().Select(UserModelExtensions.ToModelConverterExpression),
};
public static readonly Func<BlogPost, BlogPostModel> ToModelConverterFunction = ToModelConverterExpression.Compile();
public static IQueryable<BlogPostModel> ToModel(this IQueryable<BlogPost> blogPosts)
{
return blogPosts.Select(ToModelConverterExpression);
}
public static IEnumerable<BlogPostModel> ToModel(this IEnumerable<BlogPost> blogPosts)
{
return blogPosts.Select(ToModelConverterFunction);
}
}
public static class UserModelExtensions
{
public static readonly Expression<Func<User, UserModel>> ToModelConverterExpression =
u =>
new UserModel
{
Name = u.Name,
Email = u.Email,
};
public static readonly Func<User, UserModel> ToModelConverterFunction = ToModelConverterExpression.Compile();
public static IQueryable<UserModel> ToModel(this IQueryable<User> users)
{
return users.Select(ToModelConverterExpression);
}
public static IEnumerable<UserModel> ToModel(this IEnumerable<User> users)
{
return users.Select(ToModelConverterFunction);
}
}
}
To test it without actually creating a database:
var blogPostsQuery = (
from p in context.BlogPosts
where p.Title.StartsWith("a")
select p).ToModel();
Console.WriteLine(((ObjectQuery)blogPostQuery).ToTraceString());