Add needs_price_update field to Purchase model

Replace fragile price change detection in Purchase.save() with a
lazy dirty flag approach. A pre_save/post_save signal pair detects
price/currency changes without extra DB queries, and convert_prices()
uses the flag to determine which purchases need conversion.

- Add needs_price_update BooleanField with db_index
- Add pre_save signal to store old price/currency values
- Add post_save signal to set needs_price_update=True when price/currency changes
- Simplify Purchase.save() to remove DB reload + comparison logic
- Remove price_or_currency_differ_from() method
- Update convert_prices() to filter on needs_price_update flag
- Extract _get_exchange_rate() and _save_converted_price() helpers
- Add tests for the new behavior

games/tests.py

@@ -1,3 +1,109 @@
-from django.test import TestCase
+from datetime import date
 
-# Create your tests here.
+from django.test import TestCase, override_settings
+
+from games.models import Game, Platform, Purchase
+from games.tasks import convert_prices
+
+
+class PurchaseNeedsPriceUpdateTest(TestCase):
+    def setUp(self):
+        self.platform = Platform.objects.create(name="PC", icon="pc", group="PC")
+        self.game = Game.objects.create(name="Test Game", platform=self.platform)
+
+    def test_new_purchase_has_needs_price_update_true(self):
+        purchase = Purchase.objects.create(
+            price=50.0,
+            price_currency="USD",
+            date_purchased=date(2025, 1, 1),
+        )
+        purchase.games.add(self.game)
+        self.assertTrue(purchase.needs_price_update)
+
+    def test_convert_prices_sets_flag_to_false(self):
+        purchase = Purchase.objects.create(
+            price=50.0,
+            price_currency="USD",
+            date_purchased=date(2025, 1, 1),
+        )
+        purchase.games.add(self.game)
+        self.assertTrue(purchase.needs_price_update)
+
+        with override_settings(
+            CACHES={
+                "default": {
+                    "BACKEND": "django.core.cache.backends.locmem.LocMemCache"
+                }
+            }
+        ):
+            convert_prices()
+
+        purchase.refresh_from_db()
+        self.assertFalse(purchase.needs_price_update)
+
+    def test_price_change_sets_needs_price_update(self):
+        purchase = Purchase.objects.create(
+            price=50.0,
+            price_currency="USD",
+            date_purchased=date(2025, 1, 1),
+        )
+        purchase.games.add(self.game)
+        purchase.converted_price = 1000
+        purchase.converted_currency = "CZK"
+        purchase.needs_price_update = False
+        purchase.save()
+
+        purchase.price = 60.0
+        purchase.save()
+        purchase.refresh_from_db()
+        self.assertTrue(purchase.needs_price_update)
+
+    def test_currency_change_sets_needs_price_update(self):
+        purchase = Purchase.objects.create(
+            price=50.0,
+            price_currency="USD",
+            date_purchased=date(2025, 1, 1),
+        )
+        purchase.games.add(self.game)
+        purchase.converted_price = 1000
+        purchase.converted_currency = "CZK"
+        purchase.needs_price_update = False
+        purchase.save()
+
+        purchase.price_currency = "EUR"
+        purchase.save()
+        purchase.refresh_from_db()
+        self.assertTrue(purchase.needs_price_update)
+
+    def test_name_change_does_not_set_needs_price_update(self):
+        purchase = Purchase.objects.create(
+            price=50.0,
+            price_currency="USD",
+            date_purchased=date(2025, 1, 1),
+        )
+        purchase.games.add(self.game)
+        purchase.converted_price = 1000
+        purchase.converted_currency = "CZK"
+        purchase.needs_price_update = False
+        purchase.save()
+
+        purchase.name = "New Name"
+        purchase.save()
+        purchase.refresh_from_db()
+        self.assertFalse(purchase.needs_price_update)
+
+    def test_convert_prices_skips_already_converted(self):
+        purchase = Purchase.objects.create(
+            price=50.0,
+            price_currency="USD",
+            date_purchased=date(2025, 1, 1),
+        )
+        purchase.games.add(self.game)
+        purchase.converted_price = 1000
+        purchase.converted_currency = "CZK"
+        purchase.needs_price_update = False
+        purchase.save()
+
+        convert_prices()
+        purchase.refresh_from_db()
+        self.assertFalse(purchase.needs_price_update)