使用 Lambda 删除重复项

Question

我需要一些帮助，使用 lambda 表达式删除实体框架上下文中的重复条目。我有一个包含以下列的表格：

Id, DateOfIncident, Description, EmployeeId, IncidentTypeId, and IsAttendanceIncident

我想删除DateOfIncident, EmployeeID, IncidentTypeID and IsAttendanceIncident are the same. 我想保留一个条目的重复条目。我知道如何在 SQL 中使用带有 CTE 的存储过程来执行此操作，但我无法弄清楚如何使用 Lambda 表达式完成此任务。

此代码返回一个不包括我的重复项的列表，但现在我如何删除不在此列表中的那些重复项？

  var query = db.Incidents.Where(x => x.IsAttendanceIncident == "Y").GroupBy(x => new { x.EmployeeId, x.DateOfIncident, x.IsAttendanceIncident })
         .Select(x => x.FirstOrDefault());

更新：

所以我继续编写自定义 IEqualityComparer。现在如何使用 id 删除我的上下文中不在我的 distinctItems 中的事件？

 static void Main(string[] args)
    {
        DALIncidents.AttendanceEntities1 db = new DALIncidents.AttendanceEntities1();

       IEnumerable<DALIncidents.Incident> distinctItems = db.Incidents.Where(c => c.IsAttendanceIncident == "Y");
       distinctItems = distinctItems.Distinct(new DALIncidents.DistinctIncidentComparer());

       Console.ReadLine(); 
    }

Answer 1

var query = db.Incidents
    .Where(x => x.IsAttendanceIncident == "Y")
    .GroupBy(x => new { x.EmployeeId, x.DateOfIncident, x.IsAttendanceIncident })

示例 1：

    .Select(x => x.FirstOrDefault());  // your original code which retrieves entities to not delete

var dupes = db.Incidents.Except( query ); // get entities to delete

示例 2：

    .SelectMany( x => x.OrderBy( y => y.Id ).Skip(1) ); // gets dupes directly

var dupes = query; // already have what we need

最后：

foreach( var dupe in dupes )
{
    db.Incidents.Remove( dupe );
}

从我之前使用的测试上下文生成的示例 SQL，其中 Person 实体与手表具有 1:N 关系：

C＃：

context.Persons.SelectMany(x => x.Watches.OrderBy(y => y.Id).Skip(1))

生成的 SQL：

SELECT 
1 AS [C1], 
[Skip1].[Id] AS [Id], 
[Skip1].[Brand] AS [Brand], 
[Skip1].[Person_Id] AS [Person_Id]
FROM  [dbo].[Persons] AS [Extent1]
CROSS APPLY  (SELECT [Project1].[Id] AS [Id], [Project1].[Brand] AS [Brand], [Project1].[Person_Id] AS [Person_Id]
    FROM ( SELECT [Project1].[Id] AS [Id], [Project1].[Brand] AS [Brand], [Project1].[Person_Id] AS [Person_Id], row_number() OVER (ORDER BY [Project1].[Id] ASC) AS [row_number]
        FROM ( SELECT 
            [Extent2].[Id] AS [Id], 
            [Extent2].[Brand] AS [Brand], 
            [Extent2].[Person_Id] AS [Person_Id]
            FROM [dbo].[Watches] AS [Extent2]
            WHERE [Extent1].[Id] = [Extent2].[Person_Id]
        )  AS [Project1]
    )  AS [Project1]
    WHERE [Project1].[row_number] > 1 ) AS [Skip1]