LINQ to Entities实现错误

Question

我有SQLite数据库的Entity Framework 4模型。模型的一部分看起来像这样

所有Id字段都有类型Guid（SQLite表DDL中的uniqueidentifier）。但是当我按照LINQ查询运行时，我得到了异常。

var query = AbonentPfrs.Select(a => 
    new 
    {
        AbPfr = a,
        Pfr = a.PfrCertificates.Select(c => c.Id), 
        Ac = a.AcCertificates.Select(c => c.Id)
    }
);
query.ToList();

当我运行它时，我得到了

System.InvalidOperationException: The type of the key field 'Id' is expected to be 'System.Guid', but the value provided is actually of type 'System.String'.

UPDATE：我发现EF生成的SQL查询确实将字符串作为其中一个实体的标识符返回。

转到数据库((query as ObjectQuery).ToTraceString())的SQL如下所示：

SELECT 
[UnionAll1].[C2] AS [C1], 
[UnionAll1].[C3] AS [C2], 
[UnionAll1].[Id] AS [C3], 
[UnionAll1].[PfrRegNumber] AS [C4], 
[UnionAll1].[PfrUnitId] AS [C5], 
[UnionAll1].[Account_Id] AS [C6], 
[UnionAll1].[ActiveCertificate_Id] AS [C7], 
[UnionAll1].[C1] AS [C8], 
[UnionAll1].[Id1] AS [C9], 
[UnionAll1].[C4] AS [C10]
FROM  (SELECT 
  CASE WHEN ([Extent2].[Id] IS NULL) THEN NULL ELSE 1 END AS [C1], 
  1 AS [C2], 
  1 AS [C3], 
  [Extent1].[Id] AS [Id], 
  [Extent1].[PfrRegNumber] AS [PfrRegNumber], 
  [Extent1].[PfrUnitId] AS [PfrUnitId], 
  [Extent1].[Account_Id] AS [Account_Id], 
  [Extent1].[ActiveCertificate_Id] AS [ActiveCertificate_Id], 
  [Extent2].[Id] AS [Id1], 
  NULL AS [C4]
  FROM  [AbonentPfrs] AS [Extent1]
  LEFT OUTER JOIN [Certificates] AS [Extent2] ON [Extent1].[Id] = [Extent2].[AbonentPfr_PfrCertificates_Certificate_Id]
UNION ALL
  SELECT 
  2 AS [C1], 
  2 AS [C2], 
  2 AS [C3], 
  [Extent3].[Id] AS [Id], 
  [Extent3].[PfrRegNumber] AS [PfrRegNumber], 
  [Extent3].[PfrUnitId] AS [PfrUnitId], 
  [Extent3].[Account_Id] AS [Account_Id], 
  [Extent3].[ActiveCertificate_Id] AS [ActiveCertificate_Id], 
  NULL AS [C4], 
  [Extent4].[Id] AS [Id1]
  FROM  [AbonentPfrs] AS [Extent3]
  INNER JOIN [Certificates] AS [Extent4] ON [Extent3].[Id] = [Extent4].[AbonentPfr_AcCertificates_Certificate_Id]) AS [UnionAll1]
ORDER BY [UnionAll1].[Id] ASC, [UnionAll1].[C1] ASC

列的最后一列是证书实体的ID。但是最后一个有类型Guid和前一个 - 字符串。如果我分别运行部分查询，那么我会得到下一个列类型（最后两个）：

1. 首先选择：Guid，Object
2. 第二个选择：Obj​​ect，Guid
3. 全联盟：Guid，Object
4. 外部选择：String，Guid

为什么外部选择返回最后一列而不是一列的字符串？

Answer 1

执行外连接时，当您的查询遇到缺失值时，行CASE WHEN ([Extent2].[Id] IS NULL) THEN NULL ELSE 1 END AS [C1]将返回C1的空值。 在.NET中，GUID是一种值类型（128位整数），因此在不跳过循环的情况下不能将其设置为null。 （有关GUID结构的无法实现的更多信息，请参阅Why isn't there a Guid.IsNullOrEmpty() method。）

由于返回的值可以为空并格式化为xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx，因此假定它是一个字符串。

Answer 2

似乎排序就是重点。如果没有排序，最后两列的结果类型是Guid和Objects。订购 - String和Guid。两个订单表达式都依赖于[Extent2].[Id]列。 SQLite不运行Guid类型，所以我认为，值被转换为String。 这是EF4和System.Data.SQLite集成的难点之一。所以我需要找到一种方法来强制EF不生成排序条款...