如何仅在相关实体上包括选定的属性 [英] How to include only selected properties on related entities

问题描述

我只能包含相关实体。

using (var context = new BloggingContext()) 
{ 
    // Load all blogs, all related posts
    var blogs1 = context.Blogs 
                       .Include(b => b.Posts) 
                       .ToList(); 
}

但是，我不需要整个BlogPost实体。我只对特定属性感兴趣，例如：

However, I don't need entire BlogPost entity. I'm interested only in particular properties, e.g:

using (var context = new BloggingContext()) 
{ 
    // Load all blogs, all and titles of related posts
    var blogs2 = context.Blogs 
                       .Include(b => b.Posts.Select(p => p.Title) //throws runtime exeption
                       .ToList(); 

    foreach(var blogPost in blogs2.SelectMany(b => b.Posts))
    {
        Console.Writeline(blogPost.Blog.Id); //I need the object graph
        Console.WriteLine(blogPost.Title); //writes title
        Console.WriteLine(blogPost.Content); //writes null
    }
}

推荐答案

您可以使用 Include 加载整个实体，也可以将所需的项目投影到。选择：

You either use Include which loads the entire entity, or you project what you need to a .Select:

var blogs2 = context.Blogs 
    .Select(x => new 
    {
        BlogName = x.BlogName, //whatever
        PostTitles = x.Post.Select(y => y.Title).ToArray()
    }) 
   .ToList(); 

或者，您可以执行以下操作：

Or, you could do something like this:

var blogs2 = context.Blogs 
    .Select(x => new 
    {
        Blog = x,
        PostTitles = x.Post.Select(y => y.Title).ToArray()
    }) 
   .ToList(); 

A Select 总是更好不需要整个孩子，因为它可以防止查询不需要的数据。

A Select is always better when you don't need the entire child, as it prevents querying unneeded data.

这篇关于如何仅在相关实体上包括选定的属性的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！