A simple solution to executing code as part of a release

app.ex

defmodule MyApp do
  use Application
  
  def start(_type, _args) do
    import Supervisor.Spec, warn: false
    
    :ok = handle_args!
    
    children = []
    Supervisor.start_link(children, strategy: :one_for_one, name: MyApp.Supervisor)
  end
  
  def config_change(_changed, _new, _removed) do
    :ok = handle_args!
    :ok
  end
  
  defp handle_args! do
    # Any -- arguments will get stripped, so pass them plain and append the flags for convenience
    raw_args = Enum.map(:init.get_plain_arguments, fn a -> "--#{a}" end)
    case OptionParser.parse(raw_args, switches: [migrate: :boolean]) do
      {opts, _, _} when is_list(opts) ->
        migrate? = get_in(opts, [:migrate]) || false
        cond do
          migrate? -> migrate!
          :else    -> :ok
        end
      _ ->
        :ok
    end
  end
  
  defp migrate! do
    core_priv_dir = "#{:code.priv_dir(:myapp)}"
    migrations_dir = Path.join([core_priv_dir, "repo", "migrations"])
    seed_script = Path.join([core_priv_dir, "repo", "seeds.exs"])
    # Run migrations
    Ecto.Migrator.run(MyApp.Repo, migrations_dir, :up, all: true)
    # Run seed script
    Code.require_file(seed_script)
    :ok
  end
end