Lambda查詢翻譯.count作爲選擇行（EF6代碼優先）

Question

我有2個簡單的表格。

郵政是主人，有n票。我想用他們的投票計數返回帖子。 這是我的代碼：

db.posts.Select(p=> 
new PostlDto{Title= p.Title, VoteCount= p.Votes.Count}) 

這是確定的，我有1個來電，獲得職位，並多次打電話給得到計數。 但問題是，它每次選擇行而不是對它們進行計數這是一個耗時的操作。 這是SQL事件探查器顯示我

exec sp_executesql N'SELECT 
    [Extent1].[Id] AS [Id], 
    [Extent1].[PostId] AS [PostId], 
    FROM [dbo].[Votes] AS [Extent1] 
    WHERE [Extent1].[PostId] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=6 

我也試圖通過在基本查詢票表走數1通話，但它也需要太長的時間，因爲它也返回行，而不是它們的數量。 ：|

db.posts 
.Include(p=>p.Votes) 
.Select(p=> 
new PostlDto{Title= p.Title, VoteCount= p.Votes.Count}) 

這是將它轉換爲

SELECT 
    [Project1].[Id] AS [Id], 
    [Project1].[Title] AS [Title], 
    [Project1].[C1] AS [C1], 
    [Project1].[Id1] AS [Id1], 
    [Project1].[Pid] AS [Pid], 
    [Project1].[PostId] AS [PostId], 
    FROM (SELECT 
     [Extent1].[Id] AS [Id], 
     [Extent1].[Title] AS [Title], 
     [Extent2].[Id] AS [Id1], 
     [Extent2].[PostId] AS [PostId], 
     CASE WHEN ([Extent2].[Id] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1] 
     FROM [dbo].[Posts] AS [Extent1] 
     LEFT OUTER JOIN [dbo].[Votes] AS [Extent2] ON [Extent1].[Id] = [Extent2].[PostId] 
    ) AS [Project1] 
    ORDER BY [Project1].[Id] ASC, [Project1].[C1] ASC 

我怎樣才能改變它來運行一個簡單的計數，而不是查詢？ ：|

這些查詢需要幾秒鐘才能運行！

Answer 1

還有其他一些事情正在發生。你的代碼進行比較，以這種非攝製：

using System; 
using System.Linq; 
using System.Collections.Generic; 
using System.ComponentModel.DataAnnotations; 
using System.ComponentModel.DataAnnotations.Schema; 
using System.Data.Entity; 
using System.Data; 
using System.Diagnostics; 

namespace Ef6Test 
{ 

    public class Post 
    { 
     public int Id { get; set; } 
     public string Title { get; set; } 
     public virtual ICollection<Vote> Votes { get; } = new HashSet<Vote>(); 
    } 
    public class Vote 
    { 
     public int Id { get; set; } 
     public Post Post { get; set; } 

    } 

    public class PostDto 
    { 
     public string Title { get; set; } 
     public int VoteCount { get; set; } 
    } 
    class Db : DbContext 
    { 


     public DbSet<Post> Posts { get; set; } 
     public DbSet<Vote> Votes { get; set; } 
     protected override void OnModelCreating(DbModelBuilder modelBuilder) 
     { 
      base.OnModelCreating(modelBuilder); 
     } 




    } 

    class Program 
    { 


     static void Main(string[] args) 
     { 



      Database.SetInitializer(new DropCreateDatabaseAlways<Db>()); 


      using (var db = new Db()) 
      { 
       db.Database.Log = m => Console.WriteLine(m); 
       db.Database.Initialize(false); 

       var posts = db.Posts.Select(p => new PostDto { Title = p.Title, VoteCount = p.Votes.Count }).ToList(); 



      } 

      Console.WriteLine("Hit any key to exit"); 
      Console.ReadKey(); 
     } 
    } 
} 

輸出

SELECT 
    [Extent1].[Id] AS [Id], 
    [Extent1].[Title] AS [Title], 
    (SELECT 
     COUNT(1) AS [A1] 
     FROM [dbo].[Votes] AS [Extent2] 
     WHERE [Extent1].[Id] = [Extent2].[Post_Id]) AS [C1] 
    FROM [dbo].[Posts] AS [Extent1]