Ecto conditional upsert

upsert.ex

defmodule App.Posts do
  # ref. https://amandasposito.com/elixir/ecto/macro/2021/01/22/macro-ecto-custom-on-conflict.html
  # ref. https://elixirforum.com/t/upsert-conditional-update-e-g-update-only-when-existing-data-is-outdated/55503/2
  
  defmacro custom_on_conflict_update_replace_all(queryable) do
    values =
      :fields
      |> Post.__schema__()
      |> Enum.map(fn f ->
        {f, quote(do: fragment(unquote("EXCLUDED.#{f}")))}
      end)

    quote(do: Ecto.Query.update(unquote(queryable), [u], set: [unquote_splicing(values)]))
  end

  def on_conflict_query do
    from(u in Post)
    |> where([u], u.updated_at < fragment("EXCLUDED.updated_at"))
    |> custom_on_conflict_update_replace_all()
  end

  def upsert(%Post{} = post) do
    post
    |> Repo.insert(
      conflict_target: [:id],
      on_conflict: on_conflict_query(),
      stale_error_field: :updated_at,
      return: true
    )
  end
end