LINQ to Entities物化錯誤

Question

我對SQLite數據庫有Entity Framework 4模型。部分模型看起來像這樣

所有Id字段的類型都是Guid（SQLite表DDL中的uniqueidentifier）。但是當我運行下面的LINQ查詢時，我得到異常。

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

當我運行它，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：我發現EF生成SQL查詢，真正返回一個字符串作爲實體的一個標識符。

行進至DB ((query as ObjectQuery).ToTraceString())的SQL是這樣的：

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 

的最後列的證書中的實體ID。但最後有Guid類型和之前的字符串。如果我單獨運行查詢的部分，然後我得到的下一列類型（FOT最後兩個）：

1. 首先選擇：GUID，對象
2. 第二個選擇：對象，的Guid
3. UNION ALL：GUID，對象
4. 外選擇：字符串的Guid

爲什麼在倒數第二個欄外select返回字符串？

Answer 1

看來，排序是重點。沒有排序，最後兩列的結果類型是Guid和Objects。訂購 - 字符串和Guid。這兩個順序表達式均取決於[Extent2].[Id]列。 SQLite不會操作Guid類型，所以我認爲，值被轉換爲String。 這是EF4和System.Data.SQLite集成的難點之一。所以我需要找到一種方法來強制EF不會生成訂購條款...

Answer 2

執行外部聯接時，行CASE WHEN ([Extent2].[Id] IS NULL) THEN NULL ELSE 1 END AS [C1]將在您的查詢遇到缺失值時返回空值C1。 在.NET中，GUID是一個值類型（128位整數），所以不能設置爲null而不跳過箍環。 （有關GUID結構的更多信息，請參見Why isn't there a Guid.IsNullOrEmpty() method）。

由於返回的值是可以爲空且格式爲xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx，因此它被假定爲字符串。