Allow to hide a product unless a specific quota is sold out (#1351)

* Allow to hide a product unless a specific quota is sold out

* Fix required property

* Add API property and copy between events

src/pretix/base/migrations/0129_auto_20190724_1548.py

@@ -0,0 +1,21 @@
+# Generated by Django 2.2.1 on 2019-07-24 15:48
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+import pretix.base.models.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0128_auto_20190715_1510'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='item',
+            name='hidden_if_available',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, to='pretixbase.Quota'),
+        ),
+    ]

src/pretix/base/models/event.py

@@ -516,6 +516,7 @@ class Event(EventMixin, LoggedModel):
         for q in Quota.objects.filter(event=other, subevent__isnull=True).prefetch_related('items', 'variations'):
             items = list(q.items.all())
             vars = list(q.variations.all())
+            oldid = q.pk
             q.pk = None
             q.event = self
             q.cached_availability_state = None
@@ -529,6 +530,7 @@ class Event(EventMixin, LoggedModel):
                     q.items.add(item_map[i.pk])
             for v in vars:
                 q.variations.add(variation_map[v.pk])
+            self.items.filter(hidden_if_available_id=oldid).update(hidden_if_available=q)
 
         question_map = {}
         for q in Question.objects.filter(event=other).prefetch_related('items', 'options'):

src/pretix/base/models/items.py

@@ -334,6 +334,17 @@ class Item(LoggedModel):
         null=True, blank=True,
         help_text=_('This product will not be sold after the given date.')
     )
+    hidden_if_available = models.ForeignKey(
+        'Quota',
+        null=True, blank=True,
+        on_delete=models.SET_NULL,
+        verbose_name=_("Only show after sellout of"),
+        help_text=_("If you select a quota here, this product will only be shown when that quota is "
+                    "unavailable. If combined with the option to hide sold-out products, this allows you to "
+                    "swap out products for more expensive ones once they are sold out. There might be a short period "
+                    "in which both products are visible while all tickets in the referenced quota are reserved, "
+                    "but not yet sold.")
+    )
     require_voucher = models.BooleanField(
         verbose_name=_('This product can only be bought using a voucher.'),
         default=False,