Sort collection by first item of sub list in Entity Framework

问题

Is it possible to sort IQueryable collection by it's sub collections?

For example I have a Books collection. I also have an Authors collection which has a column BookId, so the BookId is a foreign key in Authors table. The relation is one-to-many (one book many authors).

I need to sort the Books collection by the first author. The book is not required to have an author. Can this be done in Entity Framework and Linq?

回答1:

The answer to your concrete question (after clarification that The authors should be ordered by name, and the books collection by previously ordered list of authors) is like this:

var query = db.Books
    .OrderBy(b => b.Authors.OrderBy(a => a.Name).Select(a => a.Name).FirstOrDefault());

which would generate a single SQL query like this:

SELECT
    [Project2].[Id] AS [Id],
    [Project2].[Title] AS [Title]
    FROM ( SELECT
        [Extent1].[Id] AS [Id],
        [Extent1].[Title] AS [Title],
        (SELECT TOP (1) [Project1].[Name] AS [Name]
            FROM ( SELECT
                [Extent2].[Name] AS [Name]
                FROM [dbo].[Author] AS [Extent2]
                WHERE [Extent1].[Id] = [Extent2].[BookId]
            )  AS [Project1]
            ORDER BY [Project1].[Name] ASC) AS [C1]
        FROM [dbo].[Book] AS [Extent1]
    )  AS [Project2]
    ORDER BY [Project2].[C1] ASC

Note that the books w/o author will be the first in the order.

Another way is to use Min function:

var query = db.Books
    .OrderBy(b => b.Authors.Min(a => a.Name));

with SQL translation:

SELECT
    [Project1].[Id] AS [Id],
    [Project1].[Title] AS [Title]
    FROM ( SELECT
        [Extent1].[Id] AS [Id],
        [Extent1].[Title] AS [Title],
        (SELECT
            MIN([Extent2].[Name]) AS [A1]
            FROM [dbo].[Author] AS [Extent2]
            WHERE [Extent1].[Id] = [Extent2].[BookId]) AS [C1]
        FROM [dbo].[Book] AS [Extent1]
    )  AS [Project1]
    ORDER BY [Project1].[C1] ASC

回答2:

Do you mean something like this?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;




namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            List<Book> books = new List<Book>() {
               new Book() { name = "A", authors = new List<string>() { "AuthorA", "AuthorB","AuthorC"}},
               new Book() { name = "B", authors = new List<string>() { "AuthorD", "AuthorE","AuthorF"}},
               new Book() { name = "C", authors = new List<string>() { "AuthorG", "AuthorH","AuthorI"}},
               new Book() { name = "D", authors = new List<string>() { "AuthorJ", "AuthorK","AuthorL"}},
               new Book() { name = "E", authors = new List<string>() { "AuthorM", "AuthorN","AuthorO"}}
           };

            var orderedBook = books.OrderBy(x => x.authors.FirstOrDefault()).ToList();
        }
    }
    public class Book
    {
        public string name { get; set; }
        public List<string> authors { get; set; }
    }
}

来源：https://stackoverflow.com/questions/41209374/sort-collection-by-first-item-of-sub-list-in-entity-framework