Elixir Ecto:如何使用belongs_to和has_many编写迁移？

defmodule MyApp.User do
  use Ecto.Schema

  schema "users" do
    field :name, :string
    has_many :posts, MyApp.Post
  end
end

defmodule MyApp.Repo.Migrations.CreateUsers do
  use Ecto.Migration

  def change do
    create table(:users) do
      add :name, :string
      timestamps()
    end
  end
end

defmodule MyApp.Post do
  use Ecto.Schema

  schema "posts" do
    field :title, :string
    belongs_to :user, MyApp.User
  end
end

defmodule MyApp.Repo.Migrations.CreatePosts do
  use Ecto.Migration

  def change do
    create table(:posts) do
      add :title, :string
      add :user_id, references(:users, on_delete: :nothing)
      timestamps()
    end
  end
end