c# - 按 Entity Framework 中子列表的第一项对集合进行排序

Question

是否可以按子集合对 IQueryable 集合进行排序？

例如，我有一个 Books 集合。我还有一个 Authors 集合，其中有一列 BookId，所以 BookId 是 Authors 表中的外键.关系是一对多的(一本书很多作者)。

我需要按第一作者对 Books 集合进行排序。这本书不需要有作者。这可以在 Entity Framework 和 Linq 中完成吗？

Answer 1

你的具体问题的答案(在澄清作者应该按名字排序，而书籍收藏按先前排序的作者列表之后)是这样的:

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

这将生成一个像这样的 SQL 查询:

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

请注意，没有作者的书籍将排在第一位。

另一种方法是使用 Min 函数:

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

使用 SQL 翻译:

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