LINQ查询论坛
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…
如果您使用成员身份并且不希望在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) }
甚至更清洁：
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post时测试这段代码，我认为如果Take（1）为null则可能会抛出错误。
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，但尝试从这里开始并检查它执行的SQL查询，让我知道它是否需要优化。
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？ 你会如何优化整个查询？