Question

Entity Framework存在一些问题，在迁移过程中没有调用“种子”方法。经过相当多的努力，我把它归结为当它调用HistoryRepositor.GetLastModel（https://github.com/aspnet/EntityFramework6/blob/master/src/EntityFramework/Migrations/History/HistoryRepository.cs）时。特别是本节;

                    var baseQuery
                        = CreateHistoryQuery(context, contextKey)
                            .OrderByDescending(h => h.MigrationId);

                    var lastModel
                        = baseQuery
                            .Select(
                                s => new
                                    {
                                        s.MigrationId,
                                        s.Model,
                                        s.ProductVersion
                                    })
                            .FirstOrDefault();

'baseQuery'的值如下所示。

{SELECT 
[Project1].[MigrationId] AS [MigrationId], 
[Project1].[ContextKey] AS [ContextKey], 
[Project1].[Model] AS [Model], 
[Project1].[ProductVersion] AS [ProductVersion]
FROM ( SELECT 
    [Extent1].[MigrationId] AS [MigrationId], 
    [Extent1].[ContextKey] AS [ContextKey], 
    [Extent1].[Model] AS [Model], 
    [Extent1].[ProductVersion] AS [ProductVersion]
    FROM [__MigrationHistory] AS [Extent1]
    WHERE [Extent1].[ContextKey] = @p__linq__0
)  AS [Project1]
ORDER BY [Project1].[MigrationId] DESC}

MigrationHistory表中有两条记录，似乎是通过在即时窗口中使用'baseQuery.ToList（）'来确认的，并且顺序正确（MigrationId降序 - 这些是时间戳，因此应该是最新的记录）

baseQuery.ToList()[0];
{System.Data.Entity.Migrations.History.HistoryRow}
ContextKey: "DT.DataAccess.AppContext"
MigrationId: "201612011150460_AutomaticMigration"
Model: {byte[5615]}
ProductVersion: "6.1.0-alpha1"

baseQuery.ToList()[1];
{System.Data.Entity.Migrations.History.HistoryRow}
ContextKey: "DT.DataAccess.AppContext"
MigrationId: "201611291632453_AutomaticMigration"
Model: {byte[5592]}
ProductVersion: "6.1.3-40302"

但是，分配给lastModel的记录不是“201612011150460_AutomaticMigration”，而是“201611291632453_AutomaticMigration”，这是意外的。

lastModel = { MigrationId = "201611291632453_AutomaticMigration", Model = {byte[5592]}, ProductVersion = "6.1.3-40302" }

使用立即窗口中的baseQuery.FirstOrDefault();（或First()）;

baseQuery.FirstOrDefault();
{System.Data.Entity.Migrations.History.HistoryRow}
ContextKey: "DT.DataAccess.AppContext"
MigrationId: "201611291632453_AutomaticMigration"
Model: {byte[5592]}
ProductVersion: "6.1.3-40302"

使用baseQuery创建OrderByDescending时，这应该意味着它是固定的顺序，对吗？

Linq FirstOrDefault没有返回第一个值？

0 个答案: