如何重用實體框架中的投影？

Question

我有一個使用實體框架來獲取數據的ASP.NET MVC應用程序。

我需要傳遞他們查看之前的entites轉化爲模型。預測可能非常複雜，但爲了保持簡單：

public static IQueryable<UserModel> ToModel(this IQueryable<User> users) 
{ 
    return from user in users 
      select new UserModel 
      { 
       Name = user.Name, 
       Email = user.Email, 
      }; 
} 

這可以在控制器中使用這樣的：

return View(Repository.Users.ToModel().ToList()); 

非常好。但是如果我想在另一個裏面使用這個投影呢？例如：

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(). 
      }; 
} 

（讓我們假設博客可以有一個以上的作者，它的類型是用戶的）。

我可以以某種方式分開的預測和重用他們另一個者裏面？

Answer 1

這裏的東西，實際工作（在一個簡單的測試應用程序），只選擇所需的字段：

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); 
     } 
    } 
} 

爲了測試它，而不實際創建一個數據庫：

var blogPostsQuery = (
    from p in context.BlogPosts 
    where p.Title.StartsWith("a") 
    select p).ToModel(); 
Console.WriteLine(((ObjectQuery)blogPostQuery).ToTraceString());