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

  def change do
    create table(:books) do
      add :isbn, :isbn13
      # other fields
    end
  end
end

defmodule MyApp.Book do
  use MyApp.Web, :model

  schema "books" do
    field :isbn, ISN.ISBN13, read_after_writes: true
    # other fields
  end
end

defp deps do
  [{:isn, "~> 2.0"}]
end

Postgrex.Types.define(MyApp.PostgrexTypes, [ISN], [])

config :my_app, MyApp.Repo,
  types: MyApp.PostgrexTypes