The table of Letterbox, in a migration of the app.

$ mix letterbox.gen.migration

writes a migration that pins the latest version:

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

  def up, do: Letterbox.Migrations.up(version: 1)
  def down, do: Letterbox.Migrations.down(to: 0)
end

The versions are PhoenixMountable.Migrations: tracked in the app's database (per prefix), and run on PostgreSQL and on SQLite. A later version of Letterbox that changes the table has a new version, and mix letterbox.gen.migration writes the migration that upgrades.

Version 1 makes letterbox_inbound: a message a row (Letterbox.Inbound), one for each provider's id (a webhook delivered twice is one message).

up/1 takes :version and :prefix (the schema of the table, in PostgreSQL); down/1 takes :to and :prefix.