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分组并从两个表Linq中选择第一个

greatest-n-per-group entity-framework-core linq entity-framework

Yehia A.Salam · 技术社区 · 6 年前

0两个实体:访问者和聊天信息

Visitor包含访问者信息,ChatMessages包含实际的聊天内容。

1第一次尝试

from c in ChatMessages
orderby c.CreatedAt descending 
group c by c.VisitorId  into x
select x.First()

这让我得到了按访客id分组的最新消息列表:

很酷,特别是生成了简短的sql查询:

SELECT [t3].[test], [t3].[Id], [t3].[Message], [t3].[UserId], [t3].[VisitorId], [t3].[isDeleted] AS [IsDeleted], [t3].[CreatedAt], [t3].[CreatedBy], [t3].[LastUpdatedAt], [t3].[LastUpdatedBy], [t3].[isFromVisitor] AS [IsFromVisitor]
FROM (
    SELECT [t0].[VisitorId]
    FROM [ChatMessages] AS [t0]
    GROUP BY [t0].[VisitorId]
    ) AS [t1]
OUTER APPLY (
    SELECT TOP (1) 1 AS [test], [t2].[Id], [t2].[Message], [t2].[UserId], [t2].[VisitorId], [t2].[isDeleted], [t2].[CreatedAt], [t2].[CreatedBy], [t2].[LastUpdatedAt], [t2].[LastUpdatedBy], [t2].[isFromVisitor]
    FROM [ChatMessages] AS [t2]
    WHERE (([t1].[VisitorId] IS NULL) AND ([t2].[VisitorId] IS NULL)) OR (([t1].[VisitorId] IS NOT NULL) AND ([t2].[VisitorId] IS NOT NULL) AND ([t1].[VisitorId] = [t2].[VisitorId]))
    ORDER BY [t2].[CreatedAt] DESC
    ) AS [t3]
ORDER BY [t3].[CreatedAt] DESC

2第二次尝试,加入访客表

from c in ChatMessages
join v in Visitors on  c.VisitorId equals v.Id 
orderby c.CreatedAt descending 
group new {Message = c, Visitor = v} by c.Visitor.Id  into x
select x

产生了我想要的:

SELECT [t2].[Id] AS [Key]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
GROUP BY [t2].[Id]
GO

-- Region Parameters
DECLARE @x1 BigInt = 1
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 2
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 3
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 4
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 5
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 6
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 7
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 8
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 9
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 10
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 11
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 12
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 13
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 14
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 15
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 16
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 17
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 18
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 19
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC
GO

-- Region Parameters
DECLARE @x1 BigInt = 20
-- EndRegion
SELECT [t0].[Id], [t0].[Message], [t0].[UserId], [t0].[VisitorId], [t0].[isDeleted] AS [IsDeleted], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[isFromVisitor] AS [IsFromVisitor], [t1].[Id] AS [Id2], [t1].[Email], [t1].[Name], [t1].[Phone], [t1].[isDeleted] AS [IsDeleted2], [t1].[CreatedAt] AS [CreatedAt2], [t1].[CreatedBy] AS [CreatedBy2], [t1].[LastUpdatedAt] AS [LastUpdatedAt2], [t1].[LastUpdatedBy] AS [LastUpdatedBy2], [t1].[Fingerprint], [t1].[IP]
FROM [ChatMessages] AS [t0]
INNER JOIN [Visitors] AS [t1] ON [t0].[VisitorId] = ([t1].[Id])
LEFT OUTER JOIN [Visitors] AS [t2] ON [t2].[Id] = [t0].[VisitorId]
WHERE @x1 = [t2].[Id]
ORDER BY [t0].[CreatedAt] DESC

我不想在数据库中出现一个我不想火的查询。此外,当在asp.net核心应用程序,我得到一个例外 EF.Property called with wrong property name. ,不确定原因:

crit: converse_app.Controllers.VisitorsController[0]
      There was an error on 'GetVisitorsAsync' invocation: System.InvalidOperationException: EF.Property called with wrong property name.
         at Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
         at Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.VisitBinary(BinaryExpression binaryExpression)
         at Microsoft.EntityFrameworkCore.SqlServer.Query.Internal.SqlServerSqlTranslatingExpressionVisitor.VisitBinary(BinaryExpression binaryExpression)
         at Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.Translate(Expression expression)
         at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.WeakEntityExpandingExpressionVisitor.Expand(Expression source, MemberIdentity member)
         at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.WeakEntityExpandingExpressionVisitor.VisitMember(MemberExpression memberExpression)
         at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.WeakEntityExpandingExpressionVisitor.VisitMember(MemberExpression memberExpression)
         at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.WeakEntityExpandingExpressionVisitor.Expand(SelectExpression selectExpression, Expression lambdaBody)
         at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.RemapLambdaBody(ShapedQueryExpression shapedQueryExpression, LambdaExpression lambdaExpression)
         at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.TranslateGroupBy(ShapedQueryExpression source, LambdaExpression keySelector, LambdaExpression elementSelector, LambdaExpression resultSelector)
         at Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
         at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
         at Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
         at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
         at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
         at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
         at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
         at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass9_0`1.<Execute>b__0()
         at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
         at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
         at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
         at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.GetEnumerator()
         at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
         at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
         at converse_app.Controllers.VisitorsController.GetVisitorsAsync(Int32 pageSize, Int32 pageNumber

我使用的是.NETCore3Preview8和EFCore3Preview8,并在MSSQL上运行。

0 回复 | 直到 6 年前

Ivan Stoev 6 年前

您要标准地查找的查询在linqtoentities(EF)中用类似于(no join s、没有 GroupBy ,使用导航属性):

var query = context.Visitors
    .Select(v => new
    {
        Visitor = v,
        Message = v.VisitorChatMessages
            .OrderByDescending(m => m.CreatedAt)
            .FirstOrDefault()
    });

但这是个陷阱。产生的查询效率很低,直到现在,EF+SQL的查询效率仍然很低。

但EF Core 3.0正朝着积极的方向发展!通常(现在仍然)我不推荐使用efcore3.0的预览版(beta版),因为它们重写了整个查询翻译/处理管道,所以很多事情都不能按预期工作。

但是今天我把我的EF Core测试环境更新到efcore3.0 预览9

  SELECT [v].[Id], [v].[CreatedAt], [v].[CreatedBy], [v].[Email], [v].[Fingerprint], [v].[IP], [v].[IsDeleted], [v].[LastUpdatedAt], [v].[LastUpdatedBy], [v].[Name], [v].[Phone], [t0].[Id], [t0].[CreatedAt], [t0].[CreatedBy], [t0].[IsDeleted], [t0].[IsFromVisitor], [t0].[LastUpdatedAt], [t0].[LastUpdatedBy], [t0].[Message], [t0].[UserId], [t0].[VisitorId]
  FROM [Visitors] AS [v]
  LEFT JOIN (
      SELECT [t].[Id], [t].[CreatedAt], [t].[CreatedBy], [t].[IsDeleted], [t].[IsFromVisitor], [t].[LastUpdatedAt], [t].[LastUpdatedBy], [t].[Message], [t].[UserId], [t].[VisitorId]
      FROM (
          SELECT [c].[Id], [c].[CreatedAt], [c].[CreatedBy], [c].[IsDeleted], [c].[IsFromVisitor], [c].[LastUpdatedAt], [c].[LastUpdatedBy], [c].[Message], [c].[UserId], [c].[VisitorId], ROW_NUMBER() OVER(PARTITION BY [c].[VisitorId] ORDER BY [c].[CreatedAt] DESC) AS [row]
          FROM [ChatMessages] AS [c]
      ) AS [t]
      WHERE [t].[row] <= 1
  ) AS [t0] ON [v].[Id] = [t0].[VisitorId]

ROW_NUMBER() OVER (PARTITION BY ORDER BY) 构造。这是EF查询翻译有史以来第一次这么做。我很兴奋。干得好,英孚核心团队!

与您的第一个查询完全相同(btw在Preview9中因运行时异常而失败)

from c in context.ChatMessages
orderby c.CreatedAt descending 
group c by c.VisitorId  into x
select x.First()

但是有更多的信息

from v in context.Visitors
from c in v.VisitorChatMessages
    .OrderByDescending(c => c.CreatedAt)
    .Take(1)
orderby c.CreatedAt descending
select new
{
    Visitor = v,
    Message = c
})

生成的SQL基本上是一样的-只是 LEFT OUTER JOIN 变成 INNER JOIN ORDER BY 最后。

看起来是这样的,要想让它成功,就必须避免 分组 和使用 GroupJoin (哪个集合导航属性在LINQ to Entities查询中表示)或 SelectMany 以达到所需的分组。

Steve Py 6 年前

您应该能够通过关联获取访问者信息,而无需显式地尝试加入/分组。

很抱歉切换到流畅的语法,我真的不喜欢linql,它似乎总是被迫通过它做任何事情。。。:)

from c in ChatMessages
orderby c.CreatedAt descending 
group c by c.VisitorId  into x
select x.First()

或

var groupedMessages = context.ChatMessages
    .OrderByDescending(c => c.CreatedAt)
    .GroupBy(c => c.VisitorId)
    .First();

var groupedMessages = context.ChatMessages
    .OrderByDescending(c => c.CreatedAt)
    .GroupBy(c => c.Visitor)
    .First();

这将为您提供一个作为访问者实体的密钥,以及该访问者的消息。然而,这多少回避了一个问题,为什么?

var visitorsWithMessages = context.Visitors.Include(v => v.VisitorChatMessages);

var visitorsWithMessages = context.Visitors
   // insert .Where() clause here to filter which Visitors we care to retrieve...
   .Select(v => new VisitorViewModel
   {
      Id = v.Id,
      Name = v.Name,
      RecentChatMessages = v.VisitorChatMessages
         .OrderByDescending(c => c.CreatedAt)
         .Select(c => new ChatMessageViewModel
         {
            Id = c.Id,
            Message = c.Message,
            CreatedAt = c.CreatedAt,
            CreatedBy = c.User.UserName ?? "Anonymous"
         }).Take(10).ToList()
    }).ToList();

它使用projection和EF的映射关系来获取访问者列表和最多10条他们最近的聊天消息。我填充包含我关心的字段的简单POCO视图模型类。这些视图模型可以安全地从方法中返回或序列化到视图/API使用者,而不会有延迟加载的风险。如果我只需要数据而不需要发送到任何地方,我可以使用匿名类型来获取我关心的字段。我们不需要显式地将实体连接在一起,您只需要对故意没有映射FK关系的实体执行此操作。我们也不需要故意急于加载实体。这个 Select 将为我们需要的列组成一个优化的SQL语句。

您可以在视图中使用这些结果,并基于Visitor+消息进行渲染,或者如果您想将每个访问者的列表10个最新消息/w访问者详细信息显示为一个扁平的消息列表,则可以更深入地研究:

编辑:在 .SelectMany

var recentMessages = context.Visitors
   .SelectMany(v => v.VisitorChatMessages
      .OrderByDescending(c => c.CreatedAt)
      .Select(c => new VisitorChatMessageViewModel
      {
          Id = c.Id,
          VisitorId = c.Visitor.Id,
          Message = c.Message,
          CreatedAt = c.CreatedAt,
          CreatedBy = c.User.UserName ?? "Anonymous",
          Visitor = c.Visitor.Name
      }).Take(10)
    }).ToList();

但不知道如何在linql中做到这一点。:)

编辑:最后一个示例将给出最后10条消息及其适用的访问者详细信息。(姓名)

例如,要获取最后100条消息并按访问者分组:

var recentMessages = context.Visitors
   .SelectMany(v => v.VisitorChatMessages
      .OrderByDescending(c => c.CreatedAt)
      .Select(c => new VisitorChatMessageViewModel
      {
          Id = c.Id,
          VisitorId = c.Visitor.Id,
          Message = c.Message,
          CreatedAt = c.CreatedAt,
          CreatedBy = c.User.UserName ?? "Anonymous",
          Visitor = c.Visitor.Name // Visitor Name, or could be a ViewModel for more info about Visitor...
      }).Take(100)
    }).GroupBy(x => x.Visitor).ToList();

如果您为访问者选择VisitorViewModel而不是“”。访客姓名“然后,您的分组键将可以访问Name、Id等,无论您从关联的访问者中选择什么。