CODEWITHSUNDEEP
entity-framework-4migration
Daniil Grudzinskiy
Daniil Grudzinskiy

Reputation: 420

Migrations with EF. Where they are stored?

Yesterday i was absolutely sertain that all migrations data for EF placed in classes, placed in my solution as nested from DbMigration. But today i was dig a slightly deeper(just try to fallback to old migration with enable data loss not with nu-get and visual studio, but with code())

DbMigrator fg = new DbMigrator(new Settings() { AutomaticDataLossEnabled = true});

fg.Update("MigrationName");

And get exception, smth like "string should be truncated", those means that migrator tried to update column from big to small MaxLength attribute. So, i had excluded migration that caused this update and move this changes to migration, those create tables. The error still was occured. I got to intellitrace and it said that those(deleted) migration still was called. Looking to requests told me things like this:

SELECT [Extent1].[MigrationId] AS [MigrationId] FROM [dbo].[__MigrationHistory] AS [Extent1]

Looking to a table __MigrationsHistory and get my deleted migration there with field model that contains crypted data(don't decrypt this yet) . I was realy shocked. Does this means that all code, have written in classes is just the fake and really executed code placed here? And does anyone know, how to work with this table, register projections of migration classes to it etc. Or the once way to work with migrations is nu-get console?

Upvotes: 2

Views: 1294

Answers (1)

Goran Obradovic
Goran Obradovic

Reputation: 9051

I am not fully sure what your primary question is, so I will first try to answer last part about __MigrationHistory table.

Code in classes is not fake, your code in classes is compiled and run.

This table, however, really contains your database model, but it is not encrypted, it is compressed. The reason why Migrations API needs to store your model, is to be able to compare it against your current actual model and track changes for you (for example when you add a new property it will be able to tell what property you added and to perform automatic db migration).

In previous version of EF there was an EdmMetadata table where hash of your model was stored, and EF was able to detect if you made some changes to model by comparing stored and current model hash value. New version when migrations are enabled stores entire model as compressed blob, so it can do diff between the model that was used to create database and current model you are using, and make automatic migrations accordingly.

You should not work directly with this table, it is automatically populated by migrations API, but nuget console is not the only way to do migrations, you can check this resource for some insights how to do it from code.

Now, regarding your question from question title (where they are stored?), migrations are stored in code, in a class inheriting from DbMigration class that migrations API creates for you when you do Add-Migration command in nuget console. When you perform an migration (Update-Database), either from nuget package manager console or from code, API will compare your current model with versions in __MigrationsHistory to find initial version (if you have not specified it) and perform all migrations in between initial and target version (if not specified otherwise target is latest version).

I'm not really clear how you did exclude your migration that causes problems, as you need to migrate your database to version before that migration, and then delete and recreate all subsequent migrations from there.

Maybe you could solve your fallback to old version problem by implementing public override void Down() method in your migration that causes problems when trying to rollback? This method can be used to execute code which performs inverse any operations for migration.

Not directly related to question but worth mentioning, there is also pretty detailed tutorial here for EF CF.

Upvotes: 1

Related Questions