LINQ查询论坛

Question

我在编写这个论坛，因为我是LINQ的新手，当用户点击主页面时遇到了这个问题。我想表显示论坛的列表如下：

Forum --- Topics (count) --- Posts (count) --- LastPostUserId --- LastPostTime 

我有以下SQL表：

Forums: 
ForumId (int32), 
Title (string), 
Description (string) 

ForumThreads: 
ThreadId (int32), 
ForumId (int32), 
UserId (guid), 
Subject (string), 
Views (int32), 
CreateDate (DateTime) 

ForumPosts: 
PostId (int32), 
ThreadId (int32), 
UserId (guid), 
Post (string), 
CreateDate (datetime) 

谢谢...

Answer 1

因为如果你使用的会员，你不希望在您的dbml的aspnet_Users显示用户名：

... 
LastPostUserId = posts.OrderByDescending(p=>p.PostId).Take(1).Select(p=> Membership.GetUser(p.UserId)) 
... 

另一个变化，使您发布的样品好一点是添加orderbydescending在帖子变量： 然后你可以从SELECT子句降4次重复OrderByDescending：

from forum in Forums 
let posts = ForumPosts.Where(p => p.ForumThreads.ForumId.Equals(forum.ForumId)).OrderByDescending(p=>p.PostId) 
select new 
{ 
    Forum = forum.Title, 
    Description = forum.Description, 
    Topics = forum.ForumThreads.Count(), 
    Posts = posts.Count(), 
    LastPostId = posts.Take(1).Select(p=>p.PostId), 
    LastPostThreadId = posts.Take(1).Select(p=>p.ThreadId), 
    LastPostUserId = posts.Take(1).Select(p=>p.UserId), 
    LastPostTime = posts.Take(1).Select(p=>p.CreateDate) 
} 

甚至清洁剂：

from forum in Forums 
let posts = ForumPosts.Where(p => p.ForumThreads.ForumId.Equals(forum.ForumId)) 
let lastPost = posts.OrderByDescending(p=>p.PostId).Take(1) 
select new 
{ 
    Forum = forum.Title, 
    Description = forum.Description, 
    Topics = forum.ForumThreads.Count(), 
    Posts = posts.Count(), 
    LastPostId = lastPost.PostId, 
    LastPostThreadId = lastPost.ThreadId, 
    LastPostUserId = lastPost.UserId, 
    LastPostUserName = Membership.GetUser(lastPost.UserId), 
    LastPostTime = lastPost.CreateDate 
} 

测试时，有没有最新帖子寿这段代码，我认为，如果采取（1）为null，则可能会引发错误..

Answer 2

from forum in forums 
from posts in db.ForumPosts.Where(p => p.Thread.ForumId.Equals(forum.ForumId)) 
select new 
{ 
Forum = forum.Title, 
Topics = forum.ForumThreads.Count(), 
Posts = posts.Count(), 
LastPostBy = posts.OrderByDescending(p => p.CreateDate).FirstOrDefault(p => p.UserId), 
LastPostTime= posts.Max(p => p.CreateDate)) 
} 

未经测试ofcourse，但尝试从这里开始并检查它执行的SQL查询，并让我知道它是否需要优化。

Answer 3

这几乎是卓有成效（尽管它会产生一些可怕的SQL; - ）...）

from forum in Forums 
let posts = ForumPosts.Where(p => p.ForumThreads.ForumId.Equals(forum.ForumId)) 
select new 
{ 
    Forum = forum.Title, 
    Description = forum.Description, 
    Topics = forum.ForumThreads.Count(), 
    Posts = posts.Count(), 
    LastPostId = posts.OrderByDescending(p=>p.PostId).Take(1).Select(p=>p.PostId), 
    LastPostThreadId = posts.OrderByDescending(p=>p.PostId).Take(1).Select(p=>p.ThreadId), 
    LastPostUserId = posts.OrderByDescending(p=>p.PostId).Take(1).Select(p=>p.UserId), 
    LastPostTime = posts.OrderByDescending(p=>p.PostId).Take(1).Select(p=>p.CreateDate) 
} 

最后一件事 - 我有一个从SQL表“ForumPosts”到“Aspnet_Users”的关系，我会喜欢将Aspnet_Users.UserName列显示为LastPostUserName ...怎么做？你将如何优化整个查询？