Entity framework 分组并首先从两个表中选择Linq_Entity Framework_Linq_Entity Framework Core_Greatest N Per Group

Entity framework 分组并首先从两个表中选择Linq

entity-framework linq entity-framework-core

Entity framework 分组并首先从两个表中选择Linq,entity-framework,linq,entity-framework-core,greatest-n-per-group,Entity Framework,Linq,Entity Framework Core,Greatest N Per Group,我正在尝试使用EFCore创建一个简单的查询，返回与我交谈的人的列表，以及我们之间发送的最后一条消息（与Facebook Messenger或Whatsapp上的显示方式非常相似）。我创建了linq查询，但它生成了一个地狱般的sql查询。我正在尝试优化linq查询以生成更好的sql，下面是完整的故事： 0这两个实体：访问者和聊天信息访问者包含访问者信息，聊天信息包含实际聊天 1.初试我尝试了第一个查询，如下所示： from c in ChatMessages orderby c.Creat

我正在尝试使用EFCore创建一个简单的查询，返回与我交谈的人的列表，以及我们之间发送的最后一条消息（与Facebook Messenger或Whatsapp上的显示方式非常相似）。我创建了linq查询，但它生成了一个地狱般的sql查询。我正在尝试优化linq查询以生成更好的sql，下面是完整的故事：

0这两个实体：访问者和聊天信息访问者包含访问者信息，聊天信息包含实际聊天

1.初试我尝试了第一个查询，如下所示：

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

这使我获得了按访问者id分组的最新消息列表：

这很酷，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，也加入访客表现在我还想返回访客信息，因此我必须加入访客表：

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

这就产生了我想要的：

问题是，生成SQL查询变得非常混乱：

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

这看起来不像是我想对数据库发起的查询。此外，在asp.net核心应用程序中执行此代码时，我收到一个异常

EF.Property，调用时使用了错误的属性名称。

，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

crit:converse\u app.controller.VisitorsController[0]
“GetVisitorsAsync”调用时出错：System.InvalidOperationException:EF.使用错误的属性名称调用属性。
在Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.VisitMethodCall调用（MethodCallExpression MethodCallExpression）
位于Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.VisitBinary（二进制表达式二进制表达式）
位于Microsoft.EntityFrameworkCore.SqlServer.Query.Internal.SqlServerSqlTranslatingExpressionVisitor.VisitBinary（二进制表达式二进制表达式）
位于Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.Translate（表达式）
位于Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslationExpressionVisitor.WeakEntityExpandingExpressionVisitor.Expand（表达式源，成员身份成员）
在Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslationExpressionVisitor.WeakEntityExpandingExpressionVisitor.VisitMember（MemberExpression MemberExpression）
在Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslationExpressionVisitor.WeakEntityExpandingExpressionVisitor.VisitMember（MemberExpression MemberExpression）
在Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslationExpressionVisitor.WeakEntityExpandingExpressionVisitor.Expand（选择表达式选择表达式，表达式lambdabodbody）
在Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslationExpressionVisitor.RemapLambdaBody（ShapedQueryExpression ShapedQueryExpression，LambdaExpression LambdaExpression）
位于Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslationExpressionVisitor.TranslateGroupBy（ShapedQueryExpression源、LambdaExpression键选择器、LambdaExpression元素选择器、LambdaExpression结果选择器）
在Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslationExpressionVisitor.VisitMethodCall调用（MethodCallExpression MethodCallExpression）
位于Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslationExpressionVisitor.VisitMethodCallExpression（MethodCallExpression MethodCallExpression）
在Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslationExpressionVisitor.VisitMethodCall调用（MethodCallExpression MethodCallExpression）
位于Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslationExpressionVisitor.VisitMethodCallExpression（MethodCallExpression MethodCallExpression）
位于Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult]（表达式查询）
位于Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult]（表达式查询，布尔异步）
位于Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult]（IDatabase数据库、表达式查询、IModel模型、布尔异步）
在Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.c__DisplayClass9_0`1.b__0（）中
位于Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc]（对象缓存键，Func`1编译器）
在Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult]（表达式查询）中
在Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult]（表达式）中
在Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.GetEnumerator（）中
位于System.Collections.Generic.List`1..ctor（IEnumerable`1集合）
at System.Linq.Enumerable.ToList[TSource]（IEnumerable`1 source）
在converse_app.Controllers.VisitorsController.GetVisitorsAsync（Int32 pageSize，Int32 pageNumber

抱歉，这篇文章太长了，所以我的问题是如何优化linq查询以获得更好的sql输出，以及引发此错误的原因

我将.NET Core 3 preview8与EF Core 3 preview8一起使用，并针对MSSQL运行。

您应该能够通过关联获取访客信息，而无需明确尝试加入/group

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

您的原始查询：

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

或

按访客分组：

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，这有点回避了一个问题，为什么

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

这将加载访问者并加载他们的相关聊天信息。实体满足对数据关系的查询。尽管我们关心细节，比如

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();

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();

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();

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

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

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
})