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/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")
@@ -12,68 +13,77 @@ currency_to = "CZK"
 currency_to = currency_to.upper()
 
 
-def save_converted_info(purchase, converted_price, converted_currency):
+def _get_exchange_rate(currency_from, currency_to, year):
     logger.info(
-        f"Setting converted price of {purchase} to {converted_price} {converted_currency} (originally {purchase.price} {purchase.price_currency})"
+        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:
+            response = requests.get(
+                f"https://cdn.jsdelivr.net/npm/@fawazahmed0/currency-api@{year}-01-01/v1/currencies/{currency_from.lower()}.json"
+            )
+            response.raise_for_status()
+            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(
+                    currency_from=currency_from,
+                    currency_to=currency_to,
+                    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 = converted_currency
-    purchase.save()
+    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(
-        converted_price__isnull=True, converted_currency=""
-    )
+        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_info(purchase, purchase.price, currency_to)
+            _save_converted_price(purchase, purchase.price, needs_update)
             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()
-        logger.info(
-            f"[convert_prices]: Looking for exchange rate in database: {currency_from}->{currency_to}"
-        )
-        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"
-                )
-                response.raise_for_status()
-                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(
-                        currency_from=currency_from,
-                        currency_to=currency_to,
-                        year=year,
-                        rate=floatformat(rate, 2),
-                    )
-                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}"
-                )
+        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,
             )