How do you configure the DbContext when creating Migrations in Entity Framework Core?

Question

Is there way that dependency injection can be configured/bootstrapped when using Entity Framework's migration commands?

Entity Framework Core supports dependency injection for DbContext subclasses. This mechanism includes allowing for configuration of data access outside of of the DbContext.

For example, the following would configure EF to persist to a SQL server using a connection string retrieved from config.json

ServiceCollection services = ...

var configuration = new Configuration().AddJsonFile( "config.json" );
services.AddEntityFramework( configuration )
    .AddSqlServer()
    .AddDbContext<BillingDbContext>( config => config.UseSqlServer() );

However, the migrations commands do not know to execute this code so Add-Migration will fail for lack of a provider or lack of a connection string.

Migrations can be made to work by overriding OnConfiguring within the DbContext subclass to specify the provider and configuration string, but that gets in the way when different configuration is desired elsewhere. Ultimately keeping my the migration commands and my code both working becomes undesirably complex.

Note: My DbContext lives in a different assembly than the entry point that uses it and my solution has multiple start-up projects.

Answer 1

If you are looking for a solution to configure context for migrations, you can use this in your DBContext class:

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        if (!optionsBuilder.IsConfigured)
        {
            IConfigurationRoot configuration = new ConfigurationBuilder()
                .SetBasePath(Directory.GetCurrentDirectory())
                .AddJsonFile("appsettings.json")
                .Build();
            var connectionString = configuration.GetConnectionString("DbCoreConnectionString");
            optionsBuilder.UseSqlServer(connectionString);
        }
    }

Remember to install those two packages to have SetBasePath and AddJsonFile methods: Microsoft.Extensions.Configuration.FileExtensions

Microsoft.Extensions.Configuration.Json

Answer 2

As @bricelam commented this functionality does not yet exist in Entity Framework 7. This missing functionality is tracked by GitHub issue aspnet/EntityFramework#639

In the mean time, the easier workaround I found was to utilize a global state rather than hassle with subclassing. Not usually my first design choice but it works well for now.

In MyDbContext:

public static bool isMigration = true;

protected override void OnConfiguring( DbContextOptionsBuilder optionsBuilder )
{
    // TODO: This is messy, but needed for migrations.
    // See https://github.com/aspnet/EntityFramework/issues/639
    if ( isMigration )
    {
        optionsBuilder.UseSqlServer( "<Your Connection String Here>" );
    }
}

In Startup.ConfigureServices().

public IServiceProvider ConfigureServices( IServiceCollection services )
{
    MyContext.isMigration = false;

    var configuration = new Configuration().AddJsonFile( "config.json" );
    services.AddEntityFramework( configuration )
        .AddSqlServer()
        .AddDbContext<MyDbContext>( config => config.UseSqlServer() );
    // ...
}

(The configuration code actually lives in an Autofac Module in my case.)