MSDN Documentation

Entity Framework Migrations

Entity Framework (EF) Migrations is a powerful feature that enables you to evolve your database schema over time as your application's model changes. It allows you to manage database schema changes in a programmatic and version-controlled way. This documentation covers the core concepts, usage, and best practices for using EF Migrations.

What are EF Migrations?

EF Migrations provide a way to incrementally update your relational database schema to match your object model. When you make changes to your domain classes (entities) or your EF DbContext, you can generate migration scripts that represent these changes. These scripts can then be applied to your database to update its structure.

Key Concepts

• Migrations: These are classes that represent a single change to your database schema. Each migration is composed of an Up method (to apply the change) and a Down method (to revert the change).
• DbContext: Your EF DbContext is the central component that EF uses to interact with your database. Migrations are typically configured and managed through your DbContext.
• IDbMigrator: This interface is used internally by EF to manage the migration process.
• MigrationsHistory Table: EF creates a special table in your database (usually named __EFMigrationsHistory) to keep track of which migrations have been applied.

Getting Started with Migrations

1. Enabling Migrations

To enable Migrations for your project, you typically run the following command in the Package Manager Console:

Enable-Migrations

This command will create a Migrations folder in your project, containing an initial configuration file.

2. Creating a New Migration

After making changes to your entity classes or DbContext, you can create a new migration that captures these changes:

Add-Migration InitialCreate

Replace InitialCreate with a descriptive name for your migration. EF will compare your current model with the last applied migration and generate the C# code for the schema changes in the Up and Down methods.

3. Applying Migrations

To apply the pending migrations to your database, use the following command:

Update-Database

This will execute the Up methods of all migrations that haven't been applied yet, updating your database schema.

4. Reverting Migrations

If you need to undo a migration, you can specify the target migration name:

Update-Database -TargetMigration InitialCreate

This will execute the Down methods of migrations applied after InitialCreate.

Advanced Scenarios

• Customizing Migrations: You can manually edit the generated migration code to perform more complex schema operations or to refine the automatic changes.
• Data Seeding: Migrations can also be used to seed your database with initial data. This is typically done in the Seed method of your DbMigrationsConfiguration class.
• Multiple Databases: EF Migrations supports managing migrations for multiple databases within the same application.
• Code-First vs. Database-First: While Migrations are primarily associated with the Code-First approach, they can also be used to manage schema changes when starting with an existing database (Database-First) using tools like the EF Power Tools.

Further Reading

• EF Core Migrations Documentation
• EF6 Code-First Migrations