Add-on purchases (DLC, Season Pass, Battle Pass) previously linked to a
parent *purchase* via the `related_purchase` self-FK. When the base game
was bought inside a multi-game purchase (e.g. a bundle), there was no
per-game purchase to point at — only the whole bundle.
Replace it with a `related_game` FK (Game -> Game): an add-on belongs to
a *game*, which is unambiguous regardless of how the base game was bought.
- models: drop `related_purchase`; add `related_game`
(SET_NULL, related_name="addon_purchases"); require it for non-GAME
types in `save()`.
- forms: replace the parent-purchase picker with a flat `related_game`
game search (reusing SearchSelectWidget/_game_options); drop the now
unused related_purchase_queryset/RelatedPurchaseChoiceField.
- views/urls: remove the obsolete related_purchase_by_game endpoint.
- add_purchase.js: drop the parent-dropdown refetch; keep platform
auto-fill; retarget the type toggle to #id_related_game.
- migration 0020: add -> backfill (related_game = parent's first game by
sort_name) -> remove related_purchase.
- tests: model validation unit tests + an e2e test for the flat picker.
related_game is deliberately game->game so it can later be synced from
IGDB's parent_game without schema changes.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
