How can I disable the use of the __MigrationHistory table in Entity Framework 4.3 Code First?

问题

I'm using Entity Framework 4.3 Code First with a custom database initializer like this:

public class MyContext : DbContext
{
    public MyContext()
    {
        Database.SetInitializer(new MyContextInitializer());
    }
}

public class MyContextInitializer : CreateDatabaseIfNotExists<MyContext>
{
    protected override void Seed(MyContext context)
    {
        // Add defaults to certain tables in the database

        base.Seed(context);
    }
}

Whenever my model changes, I edit my POCO's and mappings manually and I update my database manually.

When I run my application again, I get this error:

Server Error in '/' Application.

The model backing the 'MyContext' context has changed since the database was created. Consider using Code First Migrations to update the database (http://go.microsoft.com/fwlink/?LinkId=238269).

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: The model backing the 'MyContext' context has changed since the database was created. Consider using Code First Migrations to update the database (http://go.microsoft.com/fwlink/?LinkId=238269).

Using EFProfiler, I also notice these queries being executed:

-- statement #1
SELECT [GroupBy1].[A1] AS [C1]
FROM   (SELECT COUNT(1) AS [A1]
        FROM   [dbo].[__MigrationHistory] AS [Extent1]) AS [GroupBy1]

-- statement #2
SELECT TOP (1) [Project1].[C1]          AS [C1],
               [Project1].[MigrationId] AS [MigrationId],
               [Project1].[Model]       AS [Model]
FROM   (SELECT [Extent1].[MigrationId] AS [MigrationId],
               [Extent1].[CreatedOn]   AS [CreatedOn],
               [Extent1].[Model]       AS [Model],
               1                       AS [C1]
        FROM   [dbo].[__MigrationHistory] AS [Extent1]) AS [Project1]
ORDER  BY [Project1].[CreatedOn] DESC

How can I prevent this?

回答1:

At first I was sure it was because you set the default initializer in the ctor but investigating a bit I found that the initializer isn't run when the context is created but rather when you query/add something for the first time.

The provided initializer all check model compability so you are out of luck with them. You can easily make your own initializer like this instead though:

 public class Initializer : IDatabaseInitializer<Context>
    {

        public void InitializeDatabase(Context context)
        {
            if (!context.Database.Exists())
            {
                context.Database.Create();
                Seed(context);
                context.SaveChanges();
            }
        }

        private void Seed(Context context)
        {
            throw new NotImplementedException();
        }
    }

That shouldn't check compability and if the Database is missing it will create it.

UPDATE: "Context" should be the type of your implementation of DbContext

回答2:

Pass null to System.Data.Entity.Database's

public static void SetInitializer<TContext>(
    IDatabaseInitializer<TContext> strategy
)
where TContext : DbContext

to disable initialization for your context. Don't implement IDatabaseInitializer to disable it.

https://msdn.microsoft.com/en-us/library/system.data.entity.database.setinitializer(v=vs.113).aspx

来源：https://stackoverflow.com/questions/11259079/how-can-i-disable-the-use-of-the-migrationhistory-table-in-entity-framework-4

标签

ef-code-first

entity-framework-4.3

ef-migrations