Linq加入COUNT

Question

我有2张桌子，论坛和帖子 我想要使​​用新的额外字段检索所有论坛字段：计算属于此论坛的所有帖子。

我现在有这个：

var v =(from forum in Forums
    join post in Posts on forum.ForumID equals post.Forum.ForumID 
    select new 
    {
        forum, //Need to retrieve all fields/columns from forum     
        PostCount = //count all post that belong to this forum with a condition: count it only if post.Showit==1

    }
    ).Distinct()

1. 联接必须是左连接：如果没有属于某个论坛的帖子，则应检索论坛字段，但PostCount字段应为0.
2. 结果集必须是不同的（连接给我完整的交叉......或者如何调用它）

Answer 1

我想你想要这样的东西：

from forum in Forums
// ForumID part removed from both sides: LINQ should do that for you.
// Added "into postsInForum" to get a group join
join post in Posts on forum equals post.Forum into postsInForum
select new 
{
    Forum = forum,
    // Select the number of shown posts within the forum     
    PostCount = postsInForum.Where(post => post.ShowIt == 1).Count()
}

或者（正如评论中指出的那样）你可以在Count电话中加入一个条件 - 我总是忘记它是可用的：）

from forum in Forums
// ForumID part removed from both sides: LINQ should do that for you.
// Added "into postsInForum" to get a group join
join post in Posts on forum equals post.Forum into postsInForum
select new 
{
    Forum = forum,
    // Select the number of shown posts within the forum     
    PostCount = postsInForum.Count(post => post.ShowIt == 1)
}

仅过滤“显示”帖子的另一种方法是在联接中执行此操作：

from forum in Forums
join post in Posts.Where(post => post.ShowIt == 1)
    on forum equals post.Forum into shownPostsInForum
select new 
{
    Forum = forum,
    // Select the number of shown posts within the forum     
    PostCount = shownPostsInForum.Count()
}

我相信所有这些逻辑正确，但我不知道SQL会是什么样子......

Answer 2

如果在linqtosql设计器中将论坛连接到帖子，则会创建一个可以查询的关系属性。

var query = 
  from f in db.Forums
  select new
  {
    forum = f,
    PostCount = f.Posts.Count(p => p.ShowIt == 1)
  };