Refs #145 -- Vouchers that grant discounts

src/pretix/base/migrations/0048_auto_20161129_1330.py

@@ -0,0 +1,30 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.3 on 2016-11-29 13:30
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0047_auto_20161126_1300'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='voucher',
+            name='price_mode',
+            field=models.CharField(choices=[('none', 'No effect'), ('set', 'Set product price to'), ('subtract', 'Subtract from product price'), ('percent', 'Reduce product price by (%)')], default='set', max_length=100, verbose_name='Price mode'),
+        ),
+        migrations.RenameField(
+            model_name='voucher',
+            old_name='price',
+            new_name='value',
+        ),
+        migrations.AlterField(
+            model_name='voucher',
+            name='value',
+            field=models.DecimalField(blank=True, decimal_places=2, max_digits=10, null=True, verbose_name='Voucher value'),
+        ),
+    ]

src/pretix/base/models/vouchers.py

@@ -1,3 +1,5 @@
+from decimal import Decimal
+
 from django.conf import settings
 from django.core.exceptions import ValidationError
 from django.db import models
@@ -5,6 +7,7 @@ from django.utils.crypto import get_random_string
 from django.utils.timezone import now
 from django.utils.translation import ugettext_lazy as _
 
+from ..decimal import round_decimal
 from .base import LoggedModel
 from .event import Event
 from .items import Item, ItemVariation, Quota
@@ -59,6 +62,13 @@ class Voucher(LoggedModel):
     * You need to either select a quota or an item
     * If you select an item that has variations but do not select a variation, you cannot set block_quota
     """
+    PRICE_MODES = (
+        ('none', _('No effect')),
+        ('set', _('Set product price to')),
+        ('subtract', _('Subtract from product price')),
+        ('percent', _('Reduce product price by (%)')),
+    )
+
     event = models.ForeignKey(
         Event,
         on_delete=models.CASCADE,
@@ -98,10 +108,15 @@ class Voucher(LoggedModel):
             "If activated, a holder of this voucher code can buy tickets, even if there are none left."
         )
     )
-    price = models.DecimalField(
-        verbose_name=_("Set product price to"),
+    price_mode = models.CharField(
+        verbose_name=_("Price mode"),
+        max_length=100,
+        choices=PRICE_MODES,
+        default='set'
+    )
+    value = models.DecimalField(
+        verbose_name=_("Voucher value"),
         decimal_places=2, max_digits=10, null=True, blank=True,
-        help_text=_('If empty, the product will cost its normal price.')
     )
     item = models.ForeignKey(
         Item, related_name='vouchers',
@@ -208,3 +223,19 @@ class Voucher(LoggedModel):
         if self.valid_until and self.valid_until < now():
             return False
         return True
+
+    def calculate_price(self, original_price: Decimal) -> Decimal:
+        """
+        Returns how the price given in original_price would be modified if this
+        voucher is applied, i.e. replaced by a different price or reduced by a
+        certain percentage. If the voucher does not modify the price, the
+        original price will be returned.
+        """
+        if self.value:
+            if self.price_mode == 'set':
+                return self.value
+            elif self.price_mode == 'subtract':
+                return original_price - self.value
+            elif self.price_mode == 'percent':
+                return round_decimal(original_price * (Decimal('100.00') - self.value) / Decimal('100.00'))
+        return original_price

src/pretix/base/services/cart.py

@@ -168,11 +168,10 @@ def _add_new_items(event: Event, items: List[dict],
                         err = err or error_messages['in_part']
                     quota_ok = min(quota_ok, avail[1])
 
-        if voucher and voucher.price is not None:
-            price = voucher.price
-        else:
-            price = item.default_price if variation is None else (
-                variation.default_price if variation.default_price is not None else item.default_price)
+        price = item.default_price if variation is None else (
+            variation.default_price if variation.default_price is not None else item.default_price)
+        if voucher:
+            price = voucher.calculate_price(price)
 
         if item.free_price and 'price' in i and i['price'] is not None and i['price'] != "":
             custom_price = i['price']

src/pretix/base/services/orders.py

@@ -233,8 +233,7 @@ def _check_positions(event: Event, now_dt: datetime, positions: List[CartPositio
                 err = err or error_messages['voucher_expired']
                 cp.delete()
                 continue
-            if cp.voucher.price is not None:
-                price = cp.voucher.price
+            price = cp.voucher.calculate_price(price)
 
         if price != cp.price and not (cp.item.free_price and cp.price > price):
             positions[i] = cp