Add constraint for a maximum number of a ticket per order

src/pretix/base/migrations/0052_auto_20170324_1506.py

@@ -0,0 +1,38 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.6 on 2017-03-24 15:06
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0051_auto_20170206_2027'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='invoice',
+            options={'ordering': ('invoice_no',)},
+        ),
+        migrations.AlterModelOptions(
+            name='orderposition',
+            options={'ordering': ('positionid', 'id'), 'verbose_name': 'Order position', 'verbose_name_plural': 'Order positions'},
+        ),
+        migrations.AddField(
+            model_name='item',
+            name='max_per_order',
+            field=models.IntegerField(blank=True, help_text='This product can only be bought at most this times within one order. If you keep the field empty or set it to 0, there is no special limit for this product. The limit for the maximum number of items in the whole order applies regardless.', null=True, verbose_name='Maximum times per order'),
+        ),
+        migrations.AlterField(
+            model_name='item',
+            name='allow_cancel',
+            field=models.BooleanField(default=True, help_text='If this is active and the general event settings allo wit, orders containing this product can be canceled by the user until the order is paid for. Users cannot cancel paid orders on their own and you can cancel orders at all times, regardless of this setting', verbose_name='Allow product to be canceled'),
+        ),
+        migrations.AlterField(
+            model_name='item',
+            name='default_price',
+            field=models.DecimalField(decimal_places=2, help_text='If this product has multiple variations, you can set different prices for each of the variations. If a variation does not have a special price or if you do not have variations, this price will be used.', max_digits=7, null=True, verbose_name='Default price'),
+        ),
+    ]

src/pretix/base/models/items.py

@@ -111,6 +111,8 @@ class Item(LoggedModel):
     :type hide_without_voucher: bool
     :param allow_cancel: If set to ``False``, an order with this product can not be canceled by the user.
     :type allow_cancel: bool
+    :param max_per_order: Maximum number of times this item can be in an order. None for unlimited.
+    :type max_per_order: int
     """
 
     event = models.ForeignKey(
@@ -203,6 +205,13 @@ class Item(LoggedModel):
                     'canceled by the user until the order is paid for. Users cannot cancel paid orders on their own '
                     'and you can cancel orders at all times, regardless of this setting')
     )
+    max_per_order = models.IntegerField(
+        verbose_name=_('Maximum amount per order'),
+        null=True, blank=True,
+        help_text=_('This product can only be bought at most this many times within one order. If you keep the field '
+                    'empty or set it to 0, there is no special limit for this product. The limit for the maximum '
+                    'number of items in the whole order applies regardless.')
+    )
 
     class Meta:
         verbose_name = _("Product")

src/pretix/base/services/cart.py

@@ -33,6 +33,7 @@ error_messages = {
     'in_part': _('Some of the products you selected are no longer available in '
                  'the quantity you selected. Please see below for details.'),
     'max_items': _("You cannot select more than %s items per order."),
+    'max_items_per_product': _("You cannot select more than %(max)s items of the product %(product)s."),
     'not_started': _('The presale period for this event has not yet started.'),
     'ended': _('The presale period has ended.'),
     'price_too_high': _('The entered price is to high.'),
@@ -131,6 +132,21 @@ class CartManager:
             if op.voucher and not op.voucher.applies_to(op.item, op.variation):
                 raise CartError(error_messages['voucher_invalid_item'])
 
+        if isinstance(op, self.AddOperation):
+            if op.item.max_per_order:
+                new_total = (
+                    len([1 for p in self.positions if p.item_id == op.item.pk]) +
+                    sum([_op.count for _op in self._operations
+                         if isinstance(_op, self.AddOperation) and _op.item == op.item]) +
+                    op.count -
+                    len([1 for _op in self._operations
+                         if isinstance(_op, self.RemoveOperation) and _op.position.item_id == op.item.pk])
+                )
+
+                if new_total > op.item.max_per_order:
+                    raise CartError(error_messages['max_items_per_product'], {'max': op.item.max_per_order,
+                                                                              'product': op.item.name})
+
     def _get_price(self, item: Item, variation: Optional[ItemVariation],
                    voucher: Optional[Voucher], custom_price: Optional[Decimal]):
         price = item.default_price if variation is None else (

src/pretix/base/services/orders.py

@@ -45,6 +45,8 @@ error_messages = {
                        'meantime. Please see below for details.'),
     'internal': _("An internal error occured, please try again."),
     'empty': _("Your cart is empty."),
+    'max_items_per_product': _("You cannot select more than %(max)s items of the product %(product)s. We removed the "
+                               "surplus items from your cart."),
     'busy': _('We were not able to process your request completely as the '
               'server was too busy. Please try again.'),
     'not_started': _('The presale period for this event has not yet started.'),
@@ -206,8 +208,10 @@ def _check_date(event: Event, now_dt: datetime):
 
 def _check_positions(event: Event, now_dt: datetime, positions: List[CartPosition]):
     err = None
+    errargs = None
     _check_date(event, now_dt)
 
+    products_seen = Counter()
     for i, cp in enumerate(positions):
         if not cp.item.active or (cp.variation and not cp.variation.active):
             err = err or error_messages['unavailable']
@@ -215,6 +219,14 @@ def _check_positions(event: Event, now_dt: datetime, positions: List[CartPositio
             continue
         quotas = list(cp.item.quotas.all()) if cp.variation is None else list(cp.variation.quotas.all())
 
+        products_seen[cp.item] += 1
+        if cp.item.max_per_order and products_seen[cp.item] > cp.item.max_per_order:
+            err = error_messages['max_items_per_product']
+            errargs = {'max': cp.item.max_per_order,
+                       'product': cp.item.name}
+            cp.delete()  # Sorry!
+            break
+
         if cp.voucher:
             redeemed_in_carts = CartPosition.objects.filter(
                 Q(voucher=cp.voucher) & Q(event=event) & Q(expires__gte=now_dt)
@@ -286,7 +298,7 @@ def _check_positions(event: Event, now_dt: datetime, positions: List[CartPositio
         else:
             cp.delete()  # Sorry!
     if err:
-        raise OrderError(err)
+        raise OrderError(err, errargs)
 
 
 def _create_order(event: Event, email: str, positions: List[CartPosition], now_dt: datetime,