Question

我对使用EF 5进行查询的“最佳实践”感兴趣。

例如，让我们有2个表：电影和流派有多对多的关系，所以有第三个表GenreMovie，其中MovieId和GenreId为FK。

这意味着一种类型有几部电影，但也有一部电影可以混合使用。

在客户端有2个网格，左边是流派列表，右边是电影列表。

对于选定的类型，电影列表不仅会显示该类型的电影，还会显示所有属于此类型的电影。

此选择可以在2路上完成，具有相同的结果。

int genreId = 1; （选择id = 1的流派）

使用EF关联

var q = db.Movies
      .Select(m => new
      {
          MovieId = m.MovieId,
          Title = m.Title,
          Selected = m.GenreMovies.Where(gm => gm.GenreId == genreId).Count() > 0,
      });

使用Linq join

var q = from m in db.Movies
         join gm in db.GenreMovies.Where(gm => gm.GenreId == genreId) on m.MovieId equals gm.MovieId into gms
         from gm in gms.DefaultIfEmpty()
         select new 
         {
             MovieId = m.MovieId,
             Title = m.Title,
             Selected = gm != null
         };

显然，第一种方法代码更少，看起来更简单。

但它是真的更好的方式，也是最好的做法？

因为我听说EF关联并不总能最好地转换为有关优化的SQL查询。

我还研究了Sql Profiler以查看两种方式的结果查询。我看到以下内容：（1。方式似乎用更多的行进行查询，但我不确定它是否效率较低）???

我尝试用几十万条记录进行一些测试，但两种方式都太快，看不出任何差异。

1

exec sp_executesql N'SELECT 
[Project2].[MovieId] AS [MovieId], 
[Project2].[Title] AS [Title], 
CASE WHEN ([Project2].[C1] > 0) THEN cast(1 as bit) WHEN ( NOT ([Project2].[C2] > 0)) THEN cast(0 as bit) END AS [C1]
FROM ( SELECT 
[Project1].[MovieId] AS [MovieId], 
[Project1].[Title] AS [Title], 
[Project1].[C1] AS [C1], 
(SELECT 
    COUNT(1) AS [A1]
    FROM [dbo].[GenreMovie] AS [Extent3]
    WHERE ([Project1].[MovieId] = [Extent3].[MovieId]) AND ([Extent3].[GenreId] = @p__linq__0)) AS [C2]
FROM ( SELECT TOP (1000) 
    [c].[MovieId] AS [MovieId], 
    [c].[Title] AS [Title], 
    (SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[GenreMovie] AS [Extent2]
        WHERE ([c].[MovieId] = [Extent2].[MovieId]) AND ([Extent2].[GenreId] = @p__linq__0)) AS [C1]
    FROM [dbo].[Movie] AS [c]
)  AS [Project1])     
AS [Project2]',N'@p__linq__0 int',@p__linq__0=1

2

exec sp_executesql N'SELECT 
[Limit1].[MovieId] AS [MovieId], 
[Limit1].[Title] AS [Title], 
CASE WHEN ([Limit1].[GenreMovieId] IS NOT NULL) THEN cast(1 as bit) WHEN ([Limit1].[GenreMovieId] IS NULL) THEN cast(0 as bit) END AS [C1]
FROM ( SELECT TOP (1000) 
[Extent1].[MovieId] AS [MovieId], 
[Extent1].[Title] AS [Title], 
[Extent2].[GenreMovieId] AS [GenreMovieId]
FROM  [dbo].[Movie] AS [Extent1]
LEFT OUTER JOIN [dbo].[GenreMovie] AS [Extent2] ON ([Extent2].[GenreId] = @p__linq__0) AND ([Extent1].[MovieId] = [Extent2].[MovieId]))  
AS [Limit1]',N'@p__linq__0 int',@p__linq__0=1

Answer 1

我会这样写：

var q = db.Movies
      .Select(m => new
      {
          MovieId = m.MovieId,
          Title = m.Title,
          Selected = m.GenreMovies.Any(gm => gm.GenreId == genreId),
      });

仅出于简单原因。

使用EF 5查询，关联VS Linq加入

1 个答案: