Example custom MFA for migration task

release_tasks.ex

defmodule MyApp.ReleaseTasks do

  @app :my_app
  @start_apps [:postgrex, :ecto]

  def migrate do
    pre_start()
    Enum.each([@app], &run_migrations_for/1)
    post_start()
  end

  defp pre_start do
    :ok = Application.load(@app)
    IO.puts "Ensure app and dependencies are started."
    Enum.each(@start_apps, &Application.ensure_all_started/1)
    Enum.each([MyApp.Repo], &(&1.start_link(pool_size: 1)))
  end

  defp post_start do
    IO.puts "Success!"
    :init.stop()
  end

  defp priv_dir(app), do: "#{:code.priv_dir(app)}"

  defp run_migrations_for(app) do
    IO.puts "Running migrations for #{app}"
    Ecto.Migrator.run(MyApp.Repo, migrations_path(app), :up, all: true)
  end

  defp migrations_path(app), do: Path.join([priv_dir(app), "repo", "migrations"])
end