Automatically migrate Ecto database in release

application.ex

defmodule MyApp.Application do
  # See https://hexdocs.pm/elixir/Application.html
  # for more information on OTP Applications
  @moduledoc false
  use Application

  @migrator if Mix.env() in [:prod, :staging], do: [MyApp.Migrator], else: []

  def start(_type, _args) do
    children =
      [
        MyApp.Repo,
        MyAppWeb.Telemetry,
        {Phoenix.PubSub, name: MyApp.PubSub},
        MyAppWeb.Endpoint
      ] ++ @migrator

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end

  # Tell Phoenix to update the endpoint configuration
  # whenever the application is updated.
  def config_change(changed, _new, removed) do
    MyAppWeb.Endpoint.config_change(changed, removed)
    :ok
  end
end

migrator.ex

defmodule MyApp.Migrator do
  @moduledoc """
  Run Ecto migrations upon application startup.
  Used to automatically trigger database migrations in a CD setup.
  """

  @otp_app :my_app
  @repo MyApp.Repo

  require Logger

  use Task, restart: :transient

  def start_link(arg), do: Task.start_link(__MODULE__, :run, [arg])

  def run(_) do
    Logger.info("#{inspect(__MODULE__)} is running database migrations")
    migrations_dir = :code.priv_dir(@otp_app) |> Path.join("repo/migrations")
    Ecto.Migrator.run(@repo, migrations_dir, :up, all: true)
    Logger.info("#{inspect(__MODULE__)} has finished running migrations")
  end
end