EntityFramework NotSupportedException Join 与嵌入式 Join

Question

由于某种原因，下面的代码出现了这个错误：

EntityFramework.SqlServer.dll 中出现“System.NotSupportedException”类型的异常，但未在用户代码中处理

附加信息：LINQ to Entities 无法识别该方法 'System.Linq.IQueryable1[DansBlog.Domain.BlogEntities.Post] Retrieve(System.Linq.Expressions.Expression1[System.Func`2[DansBlog.Domain.BlogEntities.Post,System.Boolean]])' 方法，并且该方法不能翻译成商店表达式。

代码：

var blogRepository = this.repositoryFactory.Create<Blog>(unitOfWork);
var userRepository = this.repositoryFactory.Create<User>(unitOfWork);
var postRepository = this.repositoryFactory.Create<Post>(unitOfWork);

var allResult = blogRepository.Retrieve().Join(
    userRepository.Retrieve(),
    b => b.UserId,
    u => u.Id,
    (blog, user) => new GUI.Models.Blogging.Blog()
    {
        Id = blog.Id,
        Content = blog.Content,
        Title = blog.Title,
        DateAdded = blog.DateAdded,
        User = new GUI.Models.Blogging.User()
        {
            UserId = user.Id,
            FirstName = user.FirstName,
            LastName = user.LastName,
            Email = user.Email,
            UserName = user.UserName
        },
        Posts =  postRepository.Retrieve(p => p.BlogId.Equals(blog.Id)).Join(
            userRepository.Retrieve(),
            p => p.UserId,
            u => u.Id,
            (post, postUser) => new GUI.Models.Blogging.Post()
                {
                    Content = post.Content,
                    DateAdded = post.DateAdded,
                    Id = post.Id,
                    User = new GUI.Models.Blogging.User()
                    {
                        UserId = postUser.Id,
                        UserName = postUser.UserName,
                        FirstName = postUser.FirstName,
                        LastName = postUser.LastName,
                        Email = postUser.Email
                }})});

Answer 1

LINQ to Entities 不支持您对 postRepository.Retrieve 的子调用。 要完成这项工作，您需要重新构建查询结构，并使用 GroupJoin 方法在查询的前面提取您的 Posts 集合，并使用匿名类型将 Posts 与 Blog 结合起来。 生成的查询结构应如下所示：

var allResult = blogRepository.Retrieve()
               .Join(userRepository.Retrieve(), b => b.UserId, u => u.Id, (blog, user) => new { blog, user})
               .GroupJoin(postRepository.Retrieve(), 
                                     .GroupJoin(userRepository.Retrieve(), p => p.UserId, pu => pu.Id, 
                                        (post, authors) => new GUI.Models.Blogging.Post()
                                        {
                                            Id = post.Id,
                                            BlogId = post.BlogId,
                                            User = authors.Select(pa => new GUI.Models.Blogging.User()
                                            {
                                                UserId = pa.Id,
                                                UserName = pa.UserName,
                                                // etc
                                            }).FirstOrDefault()
                                        }),
                      blogData => blogData.blog.Id, p => p.BlogId, (blogData, posts) => new { blogData, posts })
                 .Select(projection => new GUI.Models.Blogging.Blog()
                 {
                    Id = projection.blogData.blog.Id,
                    // etc
                    User = new GUI.Models.Blogging.User()
                    {
                        UserId = projection.blogData.user.Id,
                        UserName = projection.blogData.user.UserName,
                        // etc
                    },
                    Posts =  projection.posts
                 }
             );

请注意，使用 Method 语法编写的 LINQ 查询并不像乍看起来那样可读。在可能的情况下考虑使用Query syntax（如果没有，则回退到方法），因为它可以提高可读性并在大多数情况下使您的代码更加简洁。例如，上面的查询看起来像：

var allResult = from blog in blogRepository.Retrieve()
                join user in userRepository.Retrieve() on b.UserId equals u.Id
                join p in postRepository.Retrieve()
                                        .GroupJoin(userRepository.Retrieve(), p => p.UserId, pu => pu.Id, 
                                            (post, authors) => new GUI.Models.Blogging.Post()
                                            {
                                                Id = post.Id,
                                                BlogId = post.BlogId,
                                                User = (from pu in authors
                                                        select new GUI.Models.Blogging.User()
                                                        {
                                                            UserId = pu.Id,
                                                            UserName = pu.UserName,
                                                            // etc
                                                        }).FirstOrDefault()
                                            }),
                          on blog.Id equals p.BlogId into posts
                 select new GUI.Models.Blogging.Blog()
                     {
                        Id = blog.Id,
                        // etc
                        User = new GUI.Models.Blogging.User()
                        {
                            UserId = user.Id,
                            UserName = user.UserName,
                            // etc
                        },
                        Posts = posts
                     }
                 );