How do I eagerly Include the child and grandchild

问题:

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

1. A query to get the Customers
2. A query per Customer to get his Books
3. 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?

回答1:

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.

回答2:

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