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/migrations/0016_add_needs_price_update.py

@@ -0,0 +1,22 @@
+# Generated by Django 6.0.1 on 2026-05-12 11:57
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('games', '0015_alter_purchase_date_purchased_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='purchase',
+            name='needs_price_update',
+            field=models.BooleanField(db_index=True, default=True),
+        ),
+        migrations.RunSQL(
+            "UPDATE games_purchase SET needs_price_update = FALSE WHERE converted_price IS NOT NULL AND converted_currency != ''",
+            reverse_sql="UPDATE games_purchase SET needs_price_update = TRUE WHERE converted_price IS NOT NULL AND converted_currency != ''",
+        ),
+    ]

games/models.py

@@ -179,6 +179,7 @@ class Purchase(models.Model):
     price_currency = models.CharField(max_length=3, default="USD")
     converted_price = models.FloatField(null=True)
     converted_currency = models.CharField(max_length=3, blank=True, default="")
+    needs_price_update = models.BooleanField(default=True, db_index=True)
     price_per_game = GeneratedField(
         expression=Coalesce(F("converted_price"), F("price"), 0) / F("num_purchases"),
         output_field=models.FloatField(),
@@ -240,12 +241,6 @@ class Purchase(models.Model):
     def is_game(self):
         return self.type == self.GAME
 
-    def price_or_currency_differ_from(self, purchase_to_compare):
-        return (
-            self.price != purchase_to_compare.price
-            or self.price_currency != purchase_to_compare.price_currency
-        )
-
     def refund(self):
         self.date_refunded = timezone.now()
         self.save()
@@ -255,19 +250,6 @@ class Purchase(models.Model):
             raise ValidationError(
                 f"{self.get_type_display()} must have a related purchase."
             )
-        if self.pk is not None:
-            # Retrieve the existing instance from the database
-            existing_purchase = Purchase.objects.get(pk=self.pk)
-            # If price has changed, reset converted fields
-            if existing_purchase.price_or_currency_differ_from(self):
-                from games.tasks import currency_to
-
-                exchange_rate = get_or_create_rate(
-                    self.price_currency, currency_to, self.date_purchased.year
-                )
-                if exchange_rate:
-                    self.converted_price = floatformat(self.price * exchange_rate, 0)
-                    self.converted_currency = currency_to
         super().save(*args, **kwargs)
 
 

games/signals.py

@@ -17,6 +17,29 @@ from games.models import Game, GameStatusChange, Purchase, Session
 logger = logging.getLogger("games")
 
 
+@receiver(pre_save, sender=Purchase)
+def store_purchase_price_snapshot(sender, instance, **kwargs):
+    """Store old price values before save so we can detect changes."""
+    if instance.pk is not None:
+        try:
+            old_instance = sender.objects.get(pk=instance.pk)
+            instance._old_price = old_instance.price
+            instance._old_currency = old_instance.price_currency
+        except sender.DoesNotExist:
+            pass
+
+
+@receiver(post_save, sender=Purchase)
+def mark_needs_price_update(sender, instance, created, **kwargs):
+    """Mark purchase for price update if price or currency changed."""
+    if not created and hasattr(instance, "_old_price"):
+        if (
+            instance.price != instance._old_price
+            or instance.price_currency != instance._old_currency
+        ):
+            sender.objects.filter(pk=instance.pk).update(needs_price_update=True)
+
+
 @receiver(m2m_changed, sender=Purchase.games.through)
 def update_num_purchases(sender, instance, action, reverse, **kwargs):
     if not reverse and action.startswith("post_"):

games/tasks.py

@@ -1,6 +1,7 @@
 import logging
 
 import requests
+from django.db import models
 from django.template.defaultfilters import floatformat
 
 logger = logging.getLogger("games")
@@ -12,41 +13,18 @@ currency_to = "CZK"
 currency_to = currency_to.upper()
 
 
-def save_converted_info(purchase, converted_price, converted_currency):
-    logger.info(
-        f"Setting converted price of {purchase} to {converted_price} {converted_currency} (originally {purchase.price} {purchase.price_currency})"
-    )
-    purchase.converted_price = converted_price
-    purchase.converted_currency = converted_currency
-    purchase.save()
-
-
-def convert_prices():
-    purchases = Purchase.objects.filter(
-        converted_price__isnull=True, converted_currency=""
-    )
-    if purchases.count() == 0:
-        logger.info("[convert_prices]: No prices to convert.")
-
-    for purchase in purchases:
-        if purchase.price_currency.upper() == currency_to or purchase.price == 0:
-            save_converted_info(purchase, purchase.price, currency_to)
-            continue
-        year = purchase.date_purchased.year
-        currency_from = purchase.price_currency.upper()
-
-        exchange_rate = ExchangeRate.objects.filter(
-            currency_from=currency_from, currency_to=currency_to, year=year
-        ).first()
+def _get_exchange_rate(currency_from, currency_to, year):
     logger.info(
         f"[convert_prices]: Looking for exchange rate in database: {currency_from}->{currency_to}"
     )
+    exchange_rate = ExchangeRate.objects.filter(
+        currency_from=currency_from, currency_to=currency_to, year=year
+    ).first()
     if not exchange_rate:
         logger.info(
             f"[convert_prices]: Getting exchange rate from {currency_from} to {currency_to} for {year}..."
         )
         try:
-                # this API endpoint only accepts lowercase currency string
             response = requests.get(
                 f"https://cdn.jsdelivr.net/npm/@fawazahmed0/currency-api@{year}-01-01/v1/currencies/{currency_from.lower()}.json"
             )
@@ -54,7 +32,6 @@ def convert_prices():
             data = response.json()
             currency_from_data = data.get(currency_from.lower())
             rate = currency_from_data.get(currency_to.lower())
-
             if rate:
                 logger.info(f"[convert_prices]: Got {rate}, saving...")
                 exchange_rate = ExchangeRate.objects.create(
@@ -63,17 +40,50 @@ def convert_prices():
                     year=year,
                     rate=floatformat(rate, 2),
                 )
+                exchange_rate = exchange_rate.rate
             else:
                 logger.info("[convert_prices]: Could not get an exchange rate.")
         except requests.RequestException as e:
             logger.info(
                 f"[convert_prices]: Failed to fetch exchange rate for {currency_from}->{currency_to} in {year}: {e}"
             )
+    elif exchange_rate:
+        exchange_rate = exchange_rate.rate
+    return exchange_rate
+
+
+def _save_converted_price(purchase, converted_price, needs_update):
+    logger.info(
+        f"Setting converted price of {purchase} to {converted_price} {currency_to} (originally {purchase.price} {purchase.price_currency})"
+    )
+    purchase.converted_price = converted_price
+    purchase.converted_currency = currency_to
+    if needs_update:
+        purchase.needs_price_update = False
+    purchase.save(update_fields=["converted_price", "converted_currency", "needs_price_update"])
+
+
+def convert_prices():
+    purchases = Purchase.objects.filter(
+        models.Q(needs_price_update=True) | models.Q(converted_price__isnull=True)
+    ).distinct()
+    if purchases.count() == 0:
+        logger.info("[convert_prices]: No prices to convert.")
+        return
+
+    for purchase in purchases:
+        needs_update = purchase.needs_price_update
+        if purchase.price_currency.upper() == currency_to or purchase.price == 0:
+            _save_converted_price(purchase, purchase.price, needs_update)
+            continue
+        year = purchase.date_purchased.year
+        currency_from = purchase.price_currency.upper()
+        exchange_rate = _get_exchange_rate(currency_from, currency_to, year)
         if exchange_rate:
-            save_converted_info(
+            _save_converted_price(
                 purchase,
-                floatformat(purchase.price * exchange_rate.rate, 0),
-                currency_to,
+                floatformat(purchase.price * exchange_rate, 0),
+                needs_update,
             )
 
 

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)