Overview

• Migration automatically updates database schema when your model changes without losing any existing data or other database objects.
• It uses new database initializer called MigrateDatabaseToLatestVersion.
• There are two kinds of Migration: Automated and Code Based.

Automated Migration

• Entity framework has introduced Automated Migration so that you don’t have to maintain database migration manually in the code file.
  • You just need to run a command in Package Manger Console to accomplish this.
• Before running the application, you have to enable automated migration by running the ‘enable-migrations’ command.
  • enable-migrations –EnableAutomaticMigration:$true
• The creates an internal sealed Configuration class in the Migration folder in the project.

internal sealed class Configuration : DbMigrationsConfiguration<SchoolDBContext>
{
	public Configuration()
	{
		AutomaticMigrationsEnabled = true;
	}
	
	protected override void Seed(SchoolDataLayer.SchoolDBContext context)
	{
		//  This method will be called after migrating to the latest version.
		//  Use the DbSet<T>.AddOrUpdate() method to avoid creating duplicate seed data.
		context.People.AddOrUpdate(
			p => p.FullName,
				new Person { FullName = "Andrew Peters" },
				new Person { FullName = "Brice Lambson" },
				new Person { FullName = "Rowan Miller" }
		);		
	}
}

• You need to set the database initializer in the context class with the new db initialization strategy MigrateDatabaseToLatestVersion.

public class SchoolDBContext: DbContext
{
	public SchoolDBContext(): base("SchoolDBConnectionString")
	{
		Database.SetInitializer(new MigrateDatabaseToLatestVersion<SchoolDBContext,
			SchoolMigrationsConfiguration>("SchoolDBConnectionString"));
	}	
}

• If you run the application and look at the created database, you will find that it has created one system table __MigrationHistory along with other tables.
• This is where automated migration maintains the history of database changes.
• To handle data loss when changing the database schema, set AutomaticMigrationDataLossAllowed = true in the configuration class constructor, along with AutomaticMigrationsEnabled = true.

Code based Migration

• Code based migration is useful when you want more control on migration.
• Code-First has two commands for code based migration:

  1. Add-migration: It will scaffold the next migration for the changes you have made to your domain classes.

     Add-Migration [-Name] <String> [-Force]
     	[-ProjectName <String>] [-StartUpProjectName <String>]
     	[-ConfigurationTypeName <String>] [-ConnectionStringName <String>]
     	[-IgnoreChanges] [<CommonParameters>]
     
     Add-Migration [-Name] <String> [-Force]
     	[-ProjectName <String>] [-StartUpProjectName <String>]
     	[-ConfigurationTypeName <String>] -ConnectionString <String>
     	-ConnectionProviderName <String> [-IgnoreChanges] [<Common Parameters>]
     

  2. Update-database: It will apply pending changes to the database based on latest scaffolding code file you create using "Add-Migration" command.

     Update-Database [-SourceMigration <String>]
     	[-TargetMigration <String>] [-Script] [-Force] [-ProjectName <String>]
     	[-StartUpProjectName <String>] [-ConfigurationTypeName <String>]
     	[-ConnectionStringName <String>] [<CommonParameters>]
     
     Update-Database [-SourceMigration <String>] [-TargetMigration <String>]
     	[-Script] [-Force] [-ProjectName <String>] [-StartUpProjectName <String>]
     	[-ConfigurationTypeName <String>] -ConnectionString <String>
     	-ConnectionProviderName <String> [<CommonParameters>]
     

Rollback Database Change

• Use Update-database command with -TargetMigration parameter to roll back the database schema to any of the previous states.
• Use the "get-migration" command to see what migration has been applied.