EF Core data motion proposal. These three examples are sequential: first we have three seed data values, then four, then we update one of them.

1-Migration.cs

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "Countries",
        columns: table => new
        {
            Name = table.Column<string>(nullable: false),
            Population = table.Column<int>(nullable: false),
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_Country", x => x.Name);
        }
    ).WithRows(new [] { // Note: we could get property names from the column names
        new { Name = "Germany", Population = 81831000 },
        new { Name = "France", Population = 65447374 },
        new { Name = "Belgium", Population = 10839905 },
    });
}

protected override void Down(MigrationBuilder migrationBuilder)
{
    migrationBuilder.DropTable(
        name: "Countries");
}

1-OnModelCreating.cs

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Country>(entity =>
    {
        entity.SeedData(new Country[]
        {
            new Country { Name = "Germany", Population = 81831000 },
            new Country { Name = "France", Population = 65447374 },
            new Country { Name = "Belgium", Population = 10839905 },
        });
    });
}

2-Migration.cs

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.InsertRows(
        table: "Countries",
        rows: new [] { // Note: we don't get compiler suggestions here
            new { Name = "Netherlands", Population = 16680000 },
        });
}

protected override void Down(MigrationBuilder migrationBuilder)
{
    migrationBuilder.DeleteRows(
        table: "Countries",
        rows: new [] { "Netherlands" }); // Note: this is an array of the primary keys
}

2-OnModelCreating.cs

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Country>(entity =>
    {
        entity.SeedData(new Country[]
        {
            new Country { Name = "Germany", Population = 81831000 },
            new Country { Name = "France", Population = 65447374 },
            new Country { Name = "Belgium", Population = 10839905 },
            new Country { Name = "Netherlands", Population = 16680000 }, // Adding new entity
        });
    });
}

3-Migration.cs

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.UpdateRows(
        table: "Countries",
        rows: new [] { // Note: we automatically detect the primary key
            new { Name = "Germany", Population = 82175700 },
        });
}

protected override void Down(MigrationBuilder migrationBuilder)
{
    migrationBuilder.UpdateRows(
        table: "Countries",
        rows: new [] { // Note: we automatically detect the primary key
            new { Name = "Germany", Population = 81831000 },
        });
}

3-OnModelCreating.cs

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Country>(entity =>
    {
        entity.SeedData(new Country[]
        {
            new Country { Name = "Germany", Population = 82175700 }, // Updating population value
            new Country { Name = "France", Population = 65447374 },
            new Country { Name = "Belgium", Population = 10839905 },
            new Country { Name = "Netherlands", Population = 16680000 },
        });
    });
}

4-OnModelCreating.cs

/* countries.json file
[
    { "name": "Germany", "population": 82175700  },
    { "name": "France", "population": 65447374 },
    { "name": "Belgium", "population": 10839905 },
    { "name": "Netherlands", "population": 16680000 }
]
*/

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Country>(entity =>
    {
        var jsonCountries = File.ReadAllText(@"countries.json");
        var countries = JsonConvert.DeserializeObject<Country[]>(jsonCountries);
        entity.SeedData(countries);
    });
}

Scenarios.md

System data

I want to be able to seed static system data, for example a list of countries or states

Migrations

I want to be able to manipulate data as part of a specific migration, adding it on Up and deleting it on Down

Raw SQL

I want to be able to run raw SQL statements as part of my migration data transformations

Multiple instances

I want to be able to seed on startup even if I have multiple servers spinning up at the same time

Publishing

I want to be able to run data transformations as part of my publishing process

Simple deployment

I want to be able to run migrations as part of my application startup

Multiple environments

I want to be able to run seed methods with specific environment configurations

Static typing

I want to be able to write static typed data motion code

Automatic seeding

I want to be able to set up seeding methods that run automatically when updating my database

Programmatic API

I want to be able to run migrations from code, for example as part of my InMemory database set up

File

I want to be able to have a file (e.g. JSON) with my seed data