c# – LINQ查询以包含子表的一行中的字段

2023年12月11日 355次阅读

我正在尝试研究如何使用LINQ查询返回主实体的详细信息,以及从1对多(或0..1到多)子实体的一行中的详细信息.

这里的细节只是一个简化的例子.这更像是我正在寻找的概念.实体框架类可能定义为：

public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public string Department { get; set; }

    public ICollection<Login> Logins { get; set; }
}

public class Login
{
    public int Id { get; set; }
    public DateTime LoginTimestamp { get; set; }
    public string IpAddress { get; set; }
    public string UserAgent { get; set; }

    public virtual User User { get; set; }
}

所以开始查询,我想要的东西像：

var results = from u in Users
    select new {u.Id, u.Name, u.Email, u.Department}

到目前为止都很好.说我想添加上次登录日期和时间：

var results = from u in Users
    select new {u.Id, u.Name, u.Email, u.Department, u.Logins.Max(LoginTimestamp)}

还好.但现在我想从最新的登录记录中添加更多细节.假设我也想包括上次登录时的IP地址.我知道查询将不得不大幅改变并使用除“Max”之外的其他内容,但我不确定如何.加入？子查询？

我已经看到过做这样的事情：

var results = from u in Users
    select new {u.Id, u.Name, u.Email, u.Department, 
        u.Logins.Max(LoginTimestamp), 
        u.Logins.OrderByDescending(LoginTimestamp).FirstOrDefault().IPAddress}

…但我怀疑效率非常低,并且对于从子实体添加的每个其他列越来越多.

什么是最好/最有效的方法？

为了清楚起见,我只希望每个用户在结果集中有一行,只包含上次登录的详细信息.

谢谢

2016年6月16日编辑：

我已经构建了这个示例,并使用LINQPad来测试查询.

对于此LINQ查询：

from u in Users
    select new {u.Id, u.Name, u.Email, u.Department, 
        LastLogin = u.Logins.OrderByDescending(l => l.LoginTimestamp).FirstOrDefault().LoginTimestamp, 
        LastIP = u.Logins.OrderByDescending(l => l.LoginTimestamp).FirstOrDefault().IpAddress}

…它产生的SQL是……

SELECT 
    [Project2].[Id] AS [Id], 
    [Project2].[Name] AS [Name], 
    [Project2].[Email] AS [Email], 
    [Project2].[Department] AS [Department], 
    [Project2].[C1] AS [C1], 
    [Limit2].[IpAddress] AS [IpAddress]
    FROM   (SELECT 
        [Extent1].[Id] AS [Id], 
        [Extent1].[Name] AS [Name], 
        [Extent1].[Email] AS [Email], 
        [Extent1].[Department] AS [Department], 
        (SELECT TOP (1) [Project1].[LoginTimestamp] AS [LoginTimestamp]
            FROM ( SELECT 
                [Extent2].[LoginTimestamp] AS [LoginTimestamp]
                FROM [dbo].[Logins] AS [Extent2]
                WHERE [Extent1].[Id] = [Extent2].[User_Id]
            )  AS [Project1]
            ORDER BY [Project1].[LoginTimestamp] DESC) AS [C1]
        FROM [dbo].[Users] AS [Extent1] ) AS [Project2]
    OUTER APPLY  (SELECT TOP (1) [Project3].[IpAddress] AS [IpAddress]
        FROM ( SELECT 
            [Extent3].[LoginTimestamp] AS [LoginTimestamp], 
            [Extent3].[IpAddress] AS [IpAddress]
            FROM [dbo].[Logins] AS [Extent3]
            WHERE [Project2].[Id] = [Extent3].[User_Id]
        )  AS [Project3]
        ORDER BY [Project3].[LoginTimestamp] DESC ) AS [Limit2]

…这不是特别有效,并且它只会变得更糟,因为它似乎为(相同的)子实体的每个字段输出添加子查询.

如果我在SQL中这样做,我会使用：

SELECT Users.Id, Users.Name, Users.Email, Users.Department, Logins.LoginTimestamp, Logins.IpAddress, Logins.UserAgent
FROM (SELECT MAX(LoginTimestamp) AS LastLoginTimestamp, User_Id
          FROM Logins AS Logins_1
          GROUP BY User_Id) AS LastLogins INNER JOIN
      Logins ON LastLogins.LastLoginTimestamp = Logins.LoginTimestamp AND LastLogins.User_Id = Logins.User_Id INNER JOIN
      Users ON Logins.User_Id = Users.Id

一个子查询,您可以根据需要从子表中引入尽可能多的字段.

所以我想我的问题是如何在LINQ中复制这个SQL查询？

或者,如果通过LINQ有更有效的方法,我很乐意看到它.

任何帮助赞赏！
谢谢

最佳答案 您可以使用let来避免使用double子查询

var result = from u in Users
             let lastLogin = u.Logins.OrderByDescending(l => l.LoginTimestamp).FirstOrDefault()
             select new {u.Id, u.Name, u.Email, u.Department, 
                    LastLogin = lastLogin.LoginTimestamp,
                    LastIP = lastLogin.IpAddress};
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