Add rarity field to trade_offer instead of looking up via cards

trades/signals.py

@@ -6,22 +6,35 @@ from cards.models import Card
 from django.db.models import F
 from trades.models import TradeOfferHaveCard, TradeOfferWantCard, TradeAcceptance
 
-def check_trade_offer_rarity(instance):
+def validate_and_set_trade_offer_rarity(instance):
+    """
+    Ensures all cards on both sides share the same rarity and sets the TradeOffer.rarity
+    if it hasn't been set already.
+    """
+    # Combine cards from both sides.
     combined_cards = list(instance.have_cards.all()) + list(instance.want_cards.all())
-    # Use the normalized rarity from each card
+    if not combined_cards:
+        return
+
+    # Gather the Rarity instances from the cards.
     rarities = {card.normalized_rarity for card in combined_cards}
     if len(rarities) > 1:
         raise ValidationError("All cards in a trade offer must have the same rarity.")
 
+    # If trade offer's rarity isn't set yet, update it.
+    if instance.rarity is None:
+        instance.rarity = combined_cards[0].normalized_rarity
+        instance.save(update_fields=["rarity"])
+
 @receiver(m2m_changed, sender=TradeOffer.have_cards.through)
 def validate_have_cards_rarity(sender, instance, action, **kwargs):
     if action == "post_add":
-        check_trade_offer_rarity(instance)
+        validate_and_set_trade_offer_rarity(instance)
 
 @receiver(m2m_changed, sender=TradeOffer.want_cards.through)
 def validate_want_cards_rarity(sender, instance, action, **kwargs):
     if action == "post_add":
-        check_trade_offer_rarity(instance)
+        validate_and_set_trade_offer_rarity(instance)
 
 ACTIVE_STATES = [
     TradeAcceptance.AcceptanceState.ACCEPTED,