Question

以下代码：

using (var db = new Entities())
{
    db.Blogs.First().Posts.Skip(10).Take(5).ToList();
}

将生成以下SQL：

-- statement #1
SELECT TOP ( 1 ) [c].[Id] AS [Id],
             [c].[Title]          AS [Title],
             [c].[Subtitle]       AS [Subtitle],
             [c].[AllowsComments] AS [AllowsComments],
             [c].[CreatedAt]      AS [CreatedAt]
FROM [dbo].[Blogs] AS [c]

-- statement #2
SELECT [Extent1].[Id] AS [Id],
   [Extent1].[Title]    AS [Title],
   [Extent1].[Text]     AS [Text],
   [Extent1].[PostedAt] AS [PostedAt],
   [Extent1].[BlogId]   AS [BlogId],
   [Extent1].[UserId]   AS [UserId]
FROM [dbo].[Posts] AS [Extent1]
WHERE [Extent1].[BlogId] = 1 /* @EntityKeyValue1 */

（来自http://ayende.com/blog/4351/nhibernate-vs-entity-framework-4-0）

NB Skip and Take尚未转换为SQL，导致博客中的所有帖子都从数据库加载，而不仅仅是我们要求的5个。

这似乎是危险的，可怕的低效率。令人难以置信的是，是什么给出了什么？

Answer 1

发生这种情况的原因是调用First，这导致Blog对象被实现。任何进一步的遍历都需要更多的查询。

请尝试使用db.Blogs.Take(1).SelectMany(b => b.Posts).Skip(10).Take(5).ToList();在一个查询中执行此操作。您可能希望在.Take(1)之前添加某种博客排序，以确保确定性结果。

修改实际上你必须在Skip之前使用OrderBy（否则LINQ to Entities将抛出异常），这使它类似于：

db.Blogs.OrderBy(b => b.Id).Take(1) // Filter to a single blog (while remaining IQueryable) .SelectMany(b => b.Posts) // Select the blog's posts .OrderBy(p => p.PublishedDate).Skip(10).Take(5).ToList(); // Filter to the correct page of posts

Answer 2

正如他在帖子中建议的那样，您可以使用EQL来执行此查询。类似的东西：

// Create a query that takes two parameters.
string queryString =
    @"SELECT VALUE product FROM 
      AdventureWorksEntities.Products AS product 
      order by product.ListPrice SKIP @skip LIMIT @limit";

ObjectQuery<Product> productQuery =
    new ObjectQuery<Product>(queryString, context);

// Add parameters to the collection.
productQuery.Parameters.Add(new ObjectParameter("skip", 3));
productQuery.Parameters.Add(new ObjectParameter("limit", 5));

// Iterate through the collection of Contact items.
foreach (Product result in productQuery)
    Console.WriteLine("ID: {0}; Name: {1}",
    result.ProductID, result.Name);

从此处获取的代码：http://msdn.microsoft.com/en-us/library/bb738702.aspx

Answer 3

您可以尝试获取您的第一个博客并使用博客ID过滤此类帖子：

Blog blog = db.Blogs.First();
blog.posts = Posts.Where(r=>r.blogID=blog.id).Skip(10).Take(5).ToList();

跳过并采取：EF 4.1中OFFSET LIMIT的有效方法？

3 个答案: