实体框架4 - 如何从另一个表（通过外键连接）中的子元素数量上限 [英] Entity Framework 4 - How to cap the number of child elements from another table (connected via a foreign key)

问题描述

我有两个数据模型 Blog 和 Post 。 BlogId是发布表

I have two data models Blog and Post. BlogId is a foreign key on the Post table

public class Blog
{
   public int ID { get; set; }

   public string Title { get; set; }

   public virtual ICollection<Post> Posts { get; set; }

  ...  
}


public class Post
{
   public int ID { get; set; }

   public virtual int BlogId { get; set; }

   public string Title { get; set; }

  ...  
}

我的存储库很高兴，并从DB中提取所有内容。

Now this works fine and my Repository is happy and pulls everything as expected from DB.

我的问题是 - 是否有办法限制获得检索的 帖子 的数量。也许一些LINQ魔法？

My question is - Is there a way to limit the number of Posts that get retrieved. Perhaps some LINQ magic?

这是我当前在仓库中的方法：

Here is what my current method in the repository looks like:

public Business FindBlog(int id)
{
    return this.context.Get<Blog>().SingleOrDefault(x => x.ID == id);
}

推荐答案

Unfortunatelly EFv4不提供简单的方法来限制导航属性返回记录的数量。

Unfortunatelly EFv4 doesn't offer easy way to limit number of returned record for navigation property.

如果您使用 EntityObject 派生实体，您可以使用以下内容：

If you are using EntityObject derived entities you can use something like:

var blog = context.Blogs
                  .Single(b => b.Id == blogId);

var posts = blog.Posts
                .CreateSourceQuery()
                .OrderByDescending(p => p.Date)
                .Take(numberOfRecords)
                .ToList();

如果您使用的是POCO，则必须执行单独的查询（如果代理的POCO可以< a href =https://stackoverflow.com/questions/6658459/efficient-way-of-checking-if-many-to-many-relationship-exists-in-ef4-1/6661380#6661380>转换导航属性到 EntityCollection< Post> 以访问 CreateSourceQuery ）：

If you are using POCOs you must execute separate query for that (in case of proxied POCOs you can convert navigation property to EntityCollection<Post> to get access to CreateSourceQuery):

var blog = context.Blogs
                  .Single(b => b.Id == blogId);

var posts = context.Posts
                   .Where(p => p.BlogId == blogId)
                   .OrderByDescending(p => p.Date)
                   .Take(numberOfPosts)
                   .ToList();

EFv4.1和DbContext API提供了仅加载有限数量的相关实体的方法：

EFv4.1 and DbContext API offers the way to load only limited number of related entities:

var blog = context.Blogs
                  .Single(b => b.Id == blogId);

context.Entry(blog)
       .Collection(b => b.Posts)
       .Query()
       .OrderByDescending(p => p.Date)
       .Take(numberOfPosts)
       .Load();

编辑：

你可以做到在具有投影的单个查询中：

You can do it in single query with projection:

var blog = context.Blogs
                  .Where(b => b.Id == blogId)
                  .Select(b => new 
                      {  
                          Blog = b,
                          Posts = b.Posts
                                   .OrderByDescending(p => Date)
                                   .Take(numberOfRecords)
                      })
                  .SingleOrDefault()

请注意，您必须访问匿名类型的第二张表格的帖子。

Just be aware that you must access posts from second paremeter of anonymous type.

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