在实体框架中使用不同或不相等的左连接

Question

我必须构建一个查询来获取未收到的用户并提醒新帖子。

我的相关表结构如下：

帖子：postId int 用户：userId int、email varchar PostAlertUsers: postAlertUserId int, postId int, userId int

所有相关字段在表之间都有外键约束。

我在 SQL 中构建了这个查询，但找不到在 Entity Framework 中工作的方法：

SELECT u.email
FROM Users u
INNER JOIN Posts p ON p.userId != u.userId 
LEFT JOIN PostAlertUsers pu ON u.userId = pu.userId AND p.postId = pu.postId 
WHERE pu.postAlertUserId IS NULL

我编写了以下 EF 查询，但没有得到相同的结果：

from u in context.Users
join pu in context.PostAlertUsers on u.userId equals pu.userId into postAlerts
from pa in postAlerts.DefaultIfEmpty()
join p in context.Posts on pa.postId equals p.postId
where pa.userId != u.userId
select u.email;

如何使用 linq to entity 获得相同的结果。使用点语法（我不知道DbSet.Where(x => ...) 语法的正确术语）会更好。

编辑：

对于Posts 中不是来自同一用户的每个Post，我想获取在PostAlertUsers 上没有记录的所有用户。

编辑 2：

试图澄清一点：

对于每个帖子，我只想提醒用户一次其他用户的新帖子，我的例程将每小时运行一次以检查是否有人要发送消息。

我想获取尚未收到有关帖子警告的所有用户，这样它就不会在 PostAlertUsers 上记录此用户和帖子组合，但会记录来自其他用户的帖子。

示例数据：

Users
------------------------
userid | email
------------------------
1      | email1@test.com
2      | email2@test.com
3      | email3@test.com
------------------------

Posts (posts are created by users)
------------------------
postId | userId
------------------------
1      | 1
2      | 3
3      | 1
------------------------

PostAlertUsers (every time a user is notified about a new post, one record is added here)
------------------------
postId | userId
------------------------
1      | 2
1      | 3
2      | 1
------------------------

生成的查询将输出以下数据：

Result (using postId and userId to identify what user have to be notified for what post)
---------------------------------
postId | userId | email
---------------------------------
2      | 2      | email2@test.com
3      | 2      | email2@test.com
3      | 3      | email3@test.com
---------------------------------

编辑：

感谢 AD.Net，我提出了以下建议：

from u in context.Users
let posts = contexto.Posts.Where(p => p.userId != u.userId)
from p in posts
join pau in context.PostAlertUsers on u.userId equals pau.userId 
into alerts
from a in alerts.DefaultIfEmpty()
where a == null || a.postId != p.postId
orderby p.idPost
select new {
    p.postId,
    u.userId,
    u.email
}

Answer 1

编辑： 另一个角度的尝试，不确定性能，可能和cross-join一样糟糕

       from p in context.Posts
       let otherUsers = context.Users.Where(u => u.UserId != p.User.UserId)
       from u in otherUsers
       join pau in alerts on u.UserId equals pau.User.UserId into alert
       from a in alert.DefaultIfEmpty()
       where a == null || a.Post.PostId != p.PostId
       select new {p.PostId, u.Email};

这是一个 LinqPad 尝试：

void Main()
{
     var users = new List<User>{new User{UserId = 1}, new User{UserId = 2}};
            var posts = new List<Post>
            {
                new Post{PostId = 1, User = new User{UserId = 2}},
                new Post{PostId = 2, User = new User{UserId = 1}},
                new Post{PostId = 3, User = new User{UserId = 2}},
            };
            var alerts = new List<PostUserAlert>
            {
                new PostUserAlert{Post = new Post{PostId = 1}, 
                                    User = new User{UserId = 1}}
            };

            var result = (from p in posts
                let otherUsers = users.Where(u => u.UserId != p.User.UserId)
                from u in otherUsers
                join pau in alerts on u.UserId equals pau.User.UserId into alert
                from a in alert.DefaultIfEmpty()
                where a == null || a.Post.PostId != p.PostId
                select new {p.PostId, u.UserId}).ToList();

                result.Dump();
}


 class User
    {
        public int UserId { get; set; }
        public string Email { get; set; }
    }

    class Post
    {
        public int PostId { get; set; }
        public User User { get; set; }

        public List<PostUserAlert>  PostUserAlerts { get; set; }
    }

    class PostUserAlert
    {
        public Post Post { get; set; }
        public User User { get; set; }
    }
// Define other methods and classes here