LINQ to Entities物化错误

Question

我对SQLite数据库有Entity Framework 4模型。部分模型看起来像这样

所有Id字段的类型都是Guid（SQLite表DDL中的uniqueidentifier）。但是当我运行下面的LINQ查询时，我得到异常。

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，我得到

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'. 

更新：我发现EF生成SQL查询，真正返回一个字符串作为实体的一个标识符。

行进至DB ((query as ObjectQuery).ToTraceString())的SQL是这样的：

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 

的最后列的证书中的实体ID。但最后有Guid类型和之前的字符串。如果我单独运行查询的部分，然后我得到的下一列类型（FOT最后两个）：

1. 首先选择：GUID，对象
2. 第二个选择：对象，的Guid
3. UNION ALL：GUID，对象
4. 外选择：字符串的Guid

为什么在倒数第二个栏外select返回字符串？

Answer 1

看来，排序是重点。没有排序，最后两列的结果类型是Guid和Objects。订购 - 字符串和Guid。这两个顺序表达式均取决于[Extent2].[Id]列。 SQLite不会操作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不会生成订购条款...

Answer 2

执行外部联接时，行CASE WHEN ([Extent2].[Id] IS NULL) THEN NULL ELSE 1 END AS [C1]将在您的查询遇到缺失值时返回空值C1。 在.NET中，GUID是一个值类型（128位整数），所以不能设置为null而不跳过箍环。 （有关GUID结构的更多信息，请参见Why isn't there a Guid.IsNullOrEmpty() method）。

由于返回的值是可以为空且格式为xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx，因此它被假定为字符串。