Question

我正在编写这个论坛，因为我是LINQ的新手，当用户点击主页面时遇到了这个问题。我想要一个显示这样的论坛列表的表：

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

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))
}

未经测试的课程，但尝试从这里开始并检查它执行的SQL查询，并告诉我它是否需要优化。

Answer 2

如果您使用成员身份并且您不想在dbml中包含aspnet_Users，则显示用户名：

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

使发布的样本更好一点的另一个变化是在posts变量中添加orderbydescending：然后你可以从select子句中删除4次重复的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)
}

甚至更干净：

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
}

当没有最后的帖子时测试这段代码，我认为如果Take（1）为null，它可能会抛出错误。

Answer 3

这几乎可以解决问题（虽然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，我想将列Aspnet_Users.UserName显示为LastPostUserName ...怎么能这样做？您将如何优化整个查询？

LINQ查询论坛

3 个答案: