如何在 Entity Framework Code First 中急切地包含实体的子元素和孙元素？

发布于 2024-11-05 02:49:37 字数 539 浏览 1 评论 0原文

想象一下三个相关的实体（客户、书籍、作者），如下所示：

客户有很多本书

一本书有一个作者

我使用该数据打印如下报告：

Customer: Peter
  Book: To Kill a Mockingbird - Author: Harper Lee
  Book: A Tale of Two Cities - Author: Charles Dickens
Customer: Melanie
  Book: The Hobbit - Author: J. R. R. Tolkien

当我查询客户时，如预期的那样，我得到了一堆查询具有以下性质

获取客户的查询
每个客户获取其书籍的查询
每本书获取其作者的查询

我可以通过包含书籍来减少查询数量，如下所示：

varcustomers = db.Customers.Include(c => c.书籍）；

但我不知道如何加载第三层（作者）。我怎样才能做到这一点？

原文

Imagine three entities (Customer, Book, Author) related like this:

A Customer has many Books

A Book has one Author

I use that data to print a report like this:

Customer: Peter
  Book: To Kill a Mockingbird - Author: Harper Lee
  Book: A Tale of Two Cities - Author: Charles Dickens
Customer: Melanie
  Book: The Hobbit - Author: J. R. R. Tolkien

When I query for Customers I get, as expected, a bunch of queries of the following nature

A query to get the Customers
A query per Customer to get his Books
A query per Book to get its author

I can reduce the number of queries by including the books like so:

var customers = db.Customers.Include(c => c.Books);

But I don't know how to load the third level (Author). How can I do that?

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鹿童谣 2024-11-12 02:49:37

此外，没有必要使用字符串重载。此方法也适用：

var customers = db.Customers.Include(c => c.Books.Select(b => b.Author));

有关更多示例，请参阅 EF 团队博客文章：

本教程：
http://www.asp.net/entity-framework/tutorials/reading-lated-data-with-the-entity-framework-in-an-asp-net-mvc-application

Also, it isn't necessary to use the string overload. This method will work too:

var customers = db.Customers.Include(c => c.Books.Select(b => b.Author));

For more examples see the EF team blog post:
http://blogs.msdn.com/b/adonet/archive/2011/01/31/using-dbcontext-in-ef-feature-ctp5-part-6-loading-related-entities.aspx

And this tutorial:
http://www.asp.net/entity-framework/tutorials/reading-related-data-with-the-entity-framework-in-an-asp-net-mvc-application

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财迷小姐 2024-11-12 02:49:37

Include 有一个重载，它接受一个字符串，该字符串可以表示您需要的任何额外属性的完整路径：

var customers = db.Customers.Include("Books.Author");

它看起来很奇怪，因为“Author”不是书籍集合上的属性（而是一个属性）在每本书上）但它有效。试一试。

There's an overload for Include that accepts a string which can denote the full path to any extra properties you need:

var customers = db.Customers.Include("Books.Author");

It looks strange because "Author" isn't a property on a collection of books (rather a property on each individual book) but it works. Give it a whirl.

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