In my system I’m adding a second repo to connect. To do that I did the following changes:

config/config.exs

config :marketplace, Marketplace.ApRepo,
  username: "postgres",
  password: "postgres",
  hostname: "localhost",
  database: "ap_dev",
  port: 5432,
  show_sensitive_data_on_connection_error: true,
  pool_size: 10

lib/ap_repo.ex

defmodule Marketplace.ApRepo do
  use AshPostgres.Repo, otp_app: :marketplace
end

lib/application.ex

defmodule Marketplace.Application do
  def start(_, _) do
    children = [
      ...
      ApRepo
    ]
    ...
  end
end

Then I added one resource to it and generated the migration.

A new migration file was correctly added to the priv/ap_repo/migrations directory.

But, when I ran the mix ash_postgres.migrate, the migration process applied all the migrations I have in my original Repo to the ApRepo and not only that new resource I created.

I did notice this warning from the documentation:

This is only really useful if your api or apis only use a single repo. If you
have multiple repos and you want to run a single migration and/or migrate/roll
them back to different points, you will need to use the ecto specific task, mix
ecto.migrate and provide your repo name.

So, does this means that I need to start using ecto.migrate from now on or only if more than one repo uses the same api?