Initial working version with minor bugs

trades/signals.py

@@ -0,0 +1,25 @@
+from django.core.exceptions import ValidationError
+from django.db.models.signals import m2m_changed
+from django.dispatch import receiver
+from .models import TradeOffer
+from cards.models import Card
+
+def check_trade_offer_rarity(instance):
+    combined_cards = list(instance.have_cards.all()) + list(instance.want_cards.all())
+    # Map rarities 6 (Super Rare) and 7 (Special Art Rare) to a single value (here, 6)
+    rarities = {
+        card.rarity_id if card.rarity_id not in (6, 7) else 6
+        for card in combined_cards
+    }
+    if len(rarities) > 1:
+        raise ValidationError("All cards in a trade offer must have the same 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)
+
+@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)