Allow the `migration_path` and `snapshot_path` to be configured per repo.

[jimsynz]

An APS client with a number of repos in their app recently asked if it's possible to have ash_postgres honour Ecto's :priv config value. I think we should both honour that value, but also allow configuration of the migration path and snapshot path on a per-repo basis.

[zachdaniel]

AFAIK this is configurable per repo. I will look into it when I'm back at a computer though.

[zachdaniel]

@jimsynz is there a reproduction or a specific example of the issue/case where we weren't honoring this option? Looking at the code:

  def derive_migrations_path(repo) do
    config = repo.config()
    priv = config[:priv] || "priv/#{repo |> Module.split() |> List.last() |> Macro.underscore()}"
    app = Keyword.fetch!(config, :otp_app)
    Application.app_dir(app, Path.join(priv, "migrations"))

  ...

  def derive_tenant_migrations_path(repo) do
    config = repo.config()
    priv = config[:priv] || "priv/#{repo |> Module.split() |> List.last() |> Macro.underscore()}"
    app = Keyword.fetch!(config, :otp_app)
    Application.app_dir(app, Path.join(priv, "tenant_migrations"))
    ...

So it's clear we're at least attempting to honor the repo's priv config value. Will need more info on what specifically illustrated that we were not.

[zachdaniel]

nvm, we don't use it for snapshots.

[zachdaniel]

But we do allow the migrations_path and tenant_migrations_path, so from what I can tell its just missing corresponding behavior in the migration generator.

    opts[:migrations_path] || repo.config()[:migrations_path] || derive_migrations_path(repo)
  end

  def tenant_migrations_path(opts, repo) do
    opts[:migrations_path] || repo.config()[:tenant_migrations_path] ||
      derive_tenant_migrations_path(repo)
  end