Restructure our python module. A lot.

src/pretix/presale/views/__init__.py

@@ -0,0 +1,60 @@
+import uuid
+from itertools import groupby
+
+from django.db.models import Q
+
+from pretix.base.models import CartPosition
+
+
+class CartMixin:
+    def get_session_key(self):
+        if 'cart_key' in self.request.session:
+            return self.request.session.get('cart_key')
+        key = str(uuid.uuid4())
+        self.request.session['cart_key'] = key
+        return key
+
+
+class CartDisplayMixin(CartMixin):
+
+    def get_cart(self):
+        qw = Q(session=self.get_session_key())
+        if self.request.user.is_authenticated():
+            qw |= Q(user=self.request.user)
+
+        cartpos = list(CartPosition.objects.current.filter(
+            qw & Q(event=self.request.event)
+        ).order_by(
+            'item', 'variation'
+        ).select_related(
+            'item', 'variation'
+        ).prefetch_related(
+            'variation__values', 'variation__values__prop'
+        ))
+
+        # Group items of the same variation
+        # We do this by list manipulations instead of a GROUP BY query, as
+        # Django is unable to join related models in a .values() query
+        def keyfunc(pos):
+            return pos.item_id, pos.variation_id, pos.price
+
+        positions = []
+        for k, g in groupby(sorted(cartpos, key=keyfunc), key=keyfunc):
+            g = list(g)
+            group = g[0]
+            group.count = len(g)
+            group.total = group.count * group.price
+            positions.append(group)
+
+        return {
+            'positions': positions,
+            'total': sum(p.total for p in positions),
+        }
+
+
+class EventViewMixin:
+
+    def get_context_data(self, **kwargs):
+        context = super().get_context_data(**kwargs)
+        context['event'] = self.request.event
+        return context