如何对实体框架中的FK属性应用预测？ [英] How to apply projections on FK property in Entity Framework?

问题描述

使用此答案，我现在可以将我的投影逻辑存储在表达式，并在另一个预测中使用。

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

然而，当我开始在我的解决方案中实现这个问题时，我发现我不能在导航属性上使用存储的表达式，该属性是单个FK（不是集合）。

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).

以下代码演示了此问题：

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, };
    }
}

是否可以将单个FK导航属性转换为模型使用表达式？

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

推荐答案

目前可能是一个过于复杂的方式：

This is currently possible in an overly complicated manner:

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

但是，生成的SQL正确的是不必要的复杂和缓慢。据我所知，目前还没有任何方式直接得到你想要的东西，但是我一直在寻找同样的东西，我有一个概念验证补丁，用于开源的EF- 6.0，我打算提交包含，请参阅讨论主题和更改。

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.

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