Migrations fails to downgrade - entity-framework

Question

Using Add-migration, I get the following code-first migration:
public partial class MyMigration : DbMigration
{
public override void Up()
{
DropForeignKey("dbo.Attachment", "EmployeePresentationID", "dbo.L_EmployeePresentation");
DropForeignKey("dbo.Attachment", "TopicID", "dbo.Topic");
DropForeignKey("dbo.Attachment", "UploaderID", "dbo.User");
}
public override void Down()
{
AddForeignKey("dbo.Attachment", "UploaderID", "dbo.User", "ID", cascadeDelete: true);
AddForeignKey("dbo.Attachment", "TopicID", "dbo.Topic", "ID");
AddForeignKey("dbo.Attachment", "EmployeePresentationID", "dbo.L_EmployeePresentation", "ID");
}
}
The code is shortened, but not modified. While the up-migration works well, the down-migration fails:
PM> Update-Database -verbose
Using StartUp project 'My_Project'.
Using NuGet project 'My_Project'.
Specify the '-Verbose' flag to view the SQL statements being applied to the target database.
Target database is: 'My-Database' (DataSource: (LocalDb)\v11.0, Provider: System.Data.SqlClient, Origin: Configuration).
Applying explicit migrations: [201410131339480_MyMigration].
Applying explicit migration: 201410131339480_MyMigration.
IF object_id(N'[dbo].[FK_dbo.AttachedFile_dbo.L_EmpPr_EmpPrID]', N'F') IS NOT NULL
ALTER TABLE [dbo].[AttachedFile] DROP CONSTRAINT [FK_dbo.AttachedFile_dbo.L_EmpPr_EmpPrID]
IF object_id(N'[dbo].[FK_dbo.AttachedFile_dbo.Topic_TopicID]', N'F') IS NOT NULL
ALTER TABLE [dbo].[AttachedFile] DROP CONSTRAINT [FK_dbo.AttachedFile_dbo.Topic_TopicID]
IF object_id(N'[dbo].[FK_dbo.AttachedFile_dbo.User_UploaderID]', N'F') IS NOT NULL
ALTER TABLE [dbo].[AttachedFile] DROP CONSTRAINT [FK_dbo.AttachedFile_dbo.User_UploaderID]
INSERT [dbo].[__MigrationHistory]([MigrationId], [ContextKey], [Model], [ProductVersion])
VALUES (N'201410131339480_MyMigration', N'My_Project.DataLayer.DataContext',
/// LOOOOOOOONG hex-stuff
Running Seed method.
PM> Update-Database -Target tags_added -verbose
Using StartUp project 'My_Project'.
Using NuGet project 'My_Project'.
Specify the '-Verbose' flag to view the SQL statements being applied to the target database.
Target database is: 'My-Database' (DataSource: (LocalDb)\v11.0, Provider: System.Data.SqlClient, Origin: Configuration).
Reverting migrations: [201410131339480_MyMigration].
Reverting explicit migration: 201410131339480_MyMigration.
ALTER TABLE [dbo].[AttachedFile] ADD CONSTRAINT [FK_dbo.AttachedFile_dbo.User_UploaderID] FOREIGN KEY ([UploaderID]) REFERENCES [dbo].[User] ([ID]) ON DELETE CASCADE
System.Data.SqlClient.SqlException (0x80131904): Introducing FOREIGN KEY constraint 'FK_dbo.AttachedFile_dbo.User_UploaderID' on table 'AttachedFile' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
Could not create constraint. See previous errors.
....
Introducing FOREIGN KEY constraint 'FK_dbo.AttachedFile_dbo.User_UploaderID' on table 'AttachedFile' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
Could not create constraint. See previous errors.
PM>
The Error baffles me, because the database has the foreign key at the moment. If this was impossible, I shouldn't have the current database, right?
Here is the table structure:
/****** Object: Table [dbo].[AttachedFile] Script Date: 10/14/2014 14:05:55 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AttachedFile](
[ID] [int] IDENTITY(1,1) NOT NULL,
[TopicID] [int] NULL,
[Deleted] [datetime] NULL,
[DisplayName] [nvarchar](max) NOT NULL,
[SafeName] [nvarchar](max) NOT NULL,
[Extension] [nvarchar](max) NOT NULL,
[Created] [datetime] NOT NULL,
[FileSize] [int] NOT NULL,
[UploaderID] [int] NOT NULL,
[EmployeePresentationID] [int] NULL,
CONSTRAINT [PK_dbo.AttachedFile] PRIMARY KEY CLUSTERED
(
[ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[AttachedFile] WITH CHECK ADD CONSTRAINT [FK_dbo.AttachedFile_dbo.L_EmployeePresentation_EmployeePresentation_ID] FOREIGN KEY([EmployeePresentationID])
REFERENCES [dbo].[L_EmployeePresentation] ([ID])
GO
ALTER TABLE [dbo].[AttachedFile] CHECK CONSTRAINT [FK_dbo.AttachedFile_dbo.L_EmployeePresentation_EmployeePresentation_ID]
GO
ALTER TABLE [dbo].[AttachedFile] WITH CHECK ADD CONSTRAINT [FK_dbo.AttachedFile_dbo.Topic_TopicID] FOREIGN KEY([TopicID])
REFERENCES [dbo].[Topic] ([ID])
GO
ALTER TABLE [dbo].[AttachedFile] CHECK CONSTRAINT [FK_dbo.AttachedFile_dbo.Topic_TopicID]
GO
ALTER TABLE [dbo].[AttachedFile] WITH CHECK ADD CONSTRAINT [FK_dbo.AttachedFile_dbo.User_Uploader_ID] FOREIGN KEY([UploaderID])
REFERENCES [dbo].[User] ([ID])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AttachedFile] CHECK CONSTRAINT [FK_dbo.AttachedFile_dbo.User_Uploader_ID]
GO
/****** Object: Table [dbo].[User] Script Date: 10/14/2014 14:06:00 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[User](
[ID] [int] IDENTITY(1,1) NOT NULL,
[Guid] [uniqueidentifier] NOT NULL,
[ShortName] [nvarchar](max) NOT NULL,
[LongName] [nvarchar](max) NULL,
[EmailAddress] [nvarchar](max) NULL,
[IsActive] [bit] NOT NULL,
[ActiveSession_ID] [int] NULL,
[Settings_ColorScheme] [int] NOT NULL,
[SessionReport_ID] [int] NULL,
[Settings_ReportOccasions] [int] NOT NULL,
CONSTRAINT [PK_dbo.User] PRIMARY KEY CLUSTERED
(
[ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[User] WITH CHECK ADD CONSTRAINT [FK_dbo.User_dbo.ActiveSession_ActiveSession_ID] FOREIGN KEY([ActiveSession_ID])
REFERENCES [dbo].[ActiveSession] ([ID])
GO
ALTER TABLE [dbo].[User] CHECK CONSTRAINT [FK_dbo.User_dbo.ActiveSession_ActiveSession_ID]
GO
ALTER TABLE [dbo].[User] WITH CHECK ADD CONSTRAINT [FK_dbo.User_dbo.SessionReport_SessionReport_ID] FOREIGN KEY([SessionReport_ID])
REFERENCES [dbo].[SessionReport] ([ID])
GO
ALTER TABLE [dbo].[User] CHECK CONSTRAINT [FK_dbo.User_dbo.SessionReport_SessionReport_ID]
GO
ALTER TABLE [dbo].[User] ADD DEFAULT ((0)) FOR [Settings_ColorScheme]
GO
ALTER TABLE [dbo].[User] ADD DEFAULT ((0)) FOR [Settings_ReportOccasions]
GO

Answer 1

If a foreign key on the dependent entity is not nullable, then Code
First sets cascade delete on the relationship.
Reference:
EF Cascade on delete
So your down migration is adding a cascade on delete that didn't exist before the up migration. Use the FluentAPI method .WillCascadeOnDelete(false) to specify that that the foreign key should not cascade on delete.