Enhance discount logic and shop configuration models

Refactored discount application logic in OrderItem to support multiple coupons with configurable multiplication and addition rules from ShopConfiguration. Updated DiscountCode model to use PositiveIntegerField for percent and improved validation. Extended ShopConfiguration with new fields for contact, social media, and coupon settings. Ensured ShopConfiguration singleton creation at app startup.

backend/commerce/models.py

@@ -1,7 +1,7 @@
 from django.db import models
 from django.conf import settings
 from django.utils import timezone
-#from django.utils.translation import gettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 from decimal import Decimal
 
 from configuration.models import ShopConfiguration
@@ -140,14 +140,16 @@ class Order(models.Model):
         carrier_price = self.carrier.get_price() if self.carrier else Decimal("0.0")
 
         if self.discount.exists():
-            for discount in self.discount.all():
-                total = Decimal('0.0')
+            discounts = list(self.discount.all())
 
-                # getting all prices from order items (with discount applied if valid)
-                for item in self.items.all():
-                    total = total + item.get_total_price(discount)
+            total = Decimal('0.0')
 
-                return total + carrier_price
+            # getting all prices from order items (with discount applied if valid)
+            for item in self.items.all():
+                total = total + item.get_total_price(discounts)
+
+            return total + carrier_price
+        
         else:
             total = Decimal('0.0')
             # getting all prices from order items (without discount)
@@ -255,8 +257,7 @@ class DiscountCode(models.Model):
     description = models.CharField(max_length=255, blank=True)
     
     # sleva v procentech (0–100)
-    percent = models.DecimalField(max_digits=5, decimal_places=2, help_text="Např. 10.00 = 10% sleva")
-    
+    percent = models.PositiveIntegerField(max_value=100, help_text="Procento sleva 0-100", null=True, blank=True)
     # nebo fixní částka
     amount = models.DecimalField(max_digits=10, decimal_places=2, null=True, blank=True, help_text="Fixní sleva v CZK")
 
@@ -264,9 +265,11 @@ class DiscountCode(models.Model):
     valid_to = models.DateTimeField(null=True, blank=True)
     active = models.BooleanField(default=True)
 
+    #max počet použití
     usage_limit = models.PositiveIntegerField(null=True, blank=True)
     used_count = models.PositiveIntegerField(default=0)
 
+
     specific_products = models.ManyToManyField(
         Product, blank=True, related_name="discount_codes"
     )
@@ -278,11 +281,12 @@ class DiscountCode(models.Model):
         now = timezone.now()
         if not self.active:
             return False
+        
         if self.valid_to and self.valid_to < now:
             return False
+        
         if self.usage_limit and self.used_count >= self.usage_limit:
             return False
-        return True
 
     def __str__(self):
         return f"{self.code} ({self.percent}% or {self.amount} CZK)"
@@ -295,27 +299,84 @@ class OrderItem(models.Model):
     product = models.ForeignKey("products.Product", on_delete=models.PROTECT)
     quantity = models.PositiveIntegerField(default=1)
 
-    def get_total_price(self, discount_object:DiscountCode = None):
-        #FIXME: přidat logiku pro slevové kódy
-        """Calculate total price for this item, applying discount if valid."""
+    def get_total_price(self, discounts: list[DiscountCode] = None):
+        """Vrátí celkovou cenu položky po aplikaci relevantních kupónů.
 
-        if discount_object and discount_object.is_valid():
+        Logika dle ShopConfiguration:
+        - multiplying_coupons=True: procentuální slevy se násobí (sekvenčně)
+          P * (1 - p1) -> výsledné * (1 - p2) ...
+          jinak se použije pouze nejlepší (nejvyšší procento).
+        - addition_of_coupons_amount=True: fixní částky (amount) se sčítají,
+          jinak se použije pouze nejvyšší částka.
+        - Kombinace: nejprve procentuální část, poté odečtení fixní částky.
+        - Sleva se nikdy nesmí dostat pod 0.
+        """
+        base_price = self.product.price * self.quantity
 
-            if (self.product in discount_object.specific_products.all() or self.product.category in discount_object.specific_categories.all()):
-                if discount_object.percent:
-                    return (self.quantity * self.product.price) * (Decimal('1.0') - discount_object.percent / Decimal('100'))
-                
-                elif discount_object.amount:
-                    return (self.quantity * self.product.price) - discount_object.amount
-                
-                else:
-                    raise ValueError("Discount code must have either percent or amount defined.")
-                
-        elif not discount_object:
-            return self.quantity * self.product.price
-        
-        else:
-            raise ValueError("Invalid discount code.")
+        if not discounts:
+            return base_price
+
+
+        config = ShopConfiguration.get_solo()
+
+        #seznám slev
+        applicable_percent_discounts: list[int] = []
+        applicable_amount_discounts: list[Decimal] = []
+
+        #procházení kupónů a určení, které se aplikují
+        for discount in set(discounts):
+            if not discount:
+                continue
+
+            if not discount.is_valid():
+                raise ValueError("Invalid discount code.")
+
+            #defaulting
+            applies = False
+
+            # Určení, zda kupon platí pro produkt/kategorii
+            # prázdný produkt a kategorie = globální kupon
+            if discount.specific_products.exists() or discount.specific_categories.exists():
+                if (self.product in discount.specific_products.all() or self.product.category in discount.specific_categories.all()):
+                    applies = True
+
+            else:
+                applies = True #global
+
+            if not applies:
+                continue
+
+            if discount.percent is not None:
+                applicable_percent_discounts.append(discount.percent)
+            elif discount.amount is not None:
+                applicable_amount_discounts.append(discount.amount)
+
+        final_price = base_price
+
+        # Procentuální slevy
+        if applicable_percent_discounts:
+
+            if config.multiplying_coupons:
+                for pct in applicable_percent_discounts:
+                    factor = (Decimal('1') - (Decimal(pct) / Decimal('100')))
+                    final_price = final_price * factor
+            else:
+                best_pct = max(applicable_percent_discounts)
+                factor = (Decimal('1') - (Decimal(best_pct) / Decimal('100')))
+                final_price = final_price * factor
+
+        # Fixní částky
+        if applicable_amount_discounts:
+            if config.addition_of_coupons_amount:
+                total_amount = sum(applicable_amount_discounts)
+            else:
+                total_amount = max(applicable_amount_discounts)
+            final_price = final_price - total_amount
+
+        if final_price < Decimal('0'):
+            final_price = Decimal('0')
+
+        return final_price.quantize(Decimal('0.01'))
 
     def __str__(self):
         return f"{self.product.name} x{self.quantity}"