C# 实体框架导航属性FirstOrDefault()
我有以下几行:C# 实体框架导航属性FirstOrDefault(),c#,sql,sql-server,linq,entity-framework-5,C#,Sql,Sql Server,Linq,Entity Framework 5,我有以下几行: WorkPlaces.FirstOrDefault() .WorkSteps.Where(x=>x.Failcodes_Id != null) .OrderByDescending(x=>x.Timestamp) .FirstOrDefault() 大约有10-20个工作场所,每个工作场所有数千个工作步骤。我想获得每个工作场所的最后一个工作步骤。 上面的代码是linqpad的一个示例,因为我无法相信生成的sql是这样的: SELECT TOP
WorkPlaces.FirstOrDefault()
.WorkSteps.Where(x=>x.Failcodes_Id != null)
.OrderByDescending(x=>x.Timestamp)
.FirstOrDefault()
大约有10-20个工作场所,每个工作场所有数千个工作步骤。我想获得每个工作场所的最后一个工作步骤。
上面的代码是linqpad的一个示例,因为我无法相信生成的sql是这样的:
SELECT TOP (1)
[Extent1].[Id] AS [Id],
[Extent1].[Name] AS [Name],
[Extent1].[Description] AS [Description],
[Extent1].[Active] AS [Active],
[Extent1].[ProductionLine_Id] AS [ProductionLine_Id],
[Extent1].[DefaultTechnology_Id] AS [DefaultTechnology_Id],
[Extent1].[PrinterName] AS [PrinterName],
[Extent1].[Deleted] AS [Deleted],
[Extent2].[Id] AS [Id1],
[Extent1].[LoggedInUser_UserId] AS [LoggedInUser_UserId]
FROM [dbo].[WorkPlaces] AS [Extent1]
LEFT OUTER JOIN [dbo].[WorkplaceParameterSet] AS [Extent2] ON [Extent1].[Id] = [Extent2].[WorkPlace_Id]
GO
-- Region Parameters
DECLARE @EntityKeyValue1 Int = 1
-- EndRegion
SELECT
[Extent1].[Id] AS [Id],
[Extent1].[Timestamp] AS [Timestamp],
[Extent1].[Description] AS [Description],
[Extent1].[WorkPlace_Id] AS [WorkPlace_Id],
[Extent1].[WorkItemState_Id] AS [WorkItemState_Id],
[Extent1].[UserId] AS [UserId],
[Extent1].[WorkItem_Id] AS [WorkItem_Id],
[Extent1].[Technology_Id] AS [Technology_Id],
[Extent1].[Failcodes_Id] AS [Failcodes_Id],
[Extent1].[DrawingNo] AS [DrawingNo],
[Extent1].[ManualData] AS [ManualData],
[Extent1].[Deleted] AS [Deleted],
[Extent1].[WorkItemState_Arrival_Id] AS [WorkItemState_Arrival_Id]
FROM [dbo].[WorkSteps] AS [Extent1]
WHERE [Extent1].[WorkPlace_Id] = @EntityKeyValue1
有没有一种方法可以在不下载9000条记录的情况下从工作步骤中获取一行以从列表顶部选择一条记录?您应该使用
IQueryable
界面,而不是IEnumerable
。也请选中,而不是单独获取每个工作场所,然后在查询中获取该工作场所的工作步骤。您可以使用选择
将每个工作场所投影到一个查询中所需的工作步骤中:
var query = WorkPlaces.Select(workplace => workplace.WorkSteps
.Where(x => x.Failcodes_Id != null)
.OrderByDescending(x => x.Timestamp)
.FirstOrDefault());
他已经是了。如果他没有,它就不会像他展示的那样生成SQL。Firstordefault->top for导航属性现在正在工作,谢谢!