The .NET Entity Framework provides functionality for automatic database migrations. Every time your application code requires a change of the database schema you should create a migration, so that the existing database schema is updated when a new version of the application is deployed. Examples for such changes are new entity classes or the addition and removal of properties of existing entity classes. The Entity Framework functionality for database migrations is called Code First Migrations.
Code First Migrations are managed via the so-called Package Manager Console. That’s how it’s called in Visual Studio, because its usually used for package management, but it’s basically a general Power Shell command line interface. After you have created the database context class and the entity model classes for your application, you create an initial migration (usually called InitialCreate), which captures the original state of the database schema for your application:
This will create a new migration class called InitialCreate in the Migrations folder. The filename is prefixed with a timestamp: 201810702207458_InitialCreate.cs. Each migration class has an Up() method, which applies the migration and a Down() method, which rolls the migration back.
Each subsequent migration only describes the difference to its predecessor migration. For example, you add a new string property Email to your User entity class and add a new migration:
The tool will scan your entity classes, compare the current state to the state of the previous migration, calculate the difference and create a new migration class, which adds a new column to the database schema.
When the migrations are run on the target system they are tracked in a special database table called __MigrationHistory.
Multiple database contexts
The above usage of Code First Migrations is well documented. Here I want to describe a feature, that is documented in less detail, because it’s less commonly used: migrations with multiple database contexts.
Let’s assume you have two database contexts: CoreDataContext and MeasurementDataContext. In this case you have to create two migration configuration classes, which inherit DbMigrationsConfiguration. You want to create two subdirectories under the Migrations directory, one for each database context:
In each subdirectory you create a migrations Configuration class, one for each database context:
internal sealed class Configuration : DbMigrationsConfiguration<CoreDataContext>
MigrationsDirectory = "Migrations\CoreData";
AutomaticMigrationsEnabled = true;
internal sealed class Configuration : DbMigrationsConfiguration<MeasruementDataContext>
MigrationsDirectory = "Migrations\MeasurementData";
AutomaticMigrationsEnabled = true;
For each configuration you have to set the MigrationsDirectory property accordingly. The AutomaticMigrationsEnabled property is optional. If it’s set the migrations will be applied automatically at the start of the application.
Now, if you run a migration command like Add-Migration, you have to add the -ConfigurationTypeName option, which specifies the Configuration class for desired the database context:
Add-Migration InitialCreate -ConfigurationTypeName Migrations.CoreData.Configuration
Add-Migration InitialCreate -ConfigurationTypeName Migrations.MeasurementData.Configuration
Add-Migration AddUserEmail -ConfigurationTypeName Migrations.CoreData.Configuration
Add-Migration AddMeasurementTimestamp -ConfigurationTypeName Migrations.MeasurementData.Configuration
The migration classes will now be created in the correct subdirectories.