Replace Item.hidden_if_available with relationship to other Item (#3686)

* draft

* Implementation that is closer to old one

* Fix tests

* Add tests

* Update src/pretix/control/forms/item.py

Co-authored-by: Richard Schreiber <schreiber@rami.io>

* Review notes

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/base/migrations/0249_hidden_if_item_available.py

@@ -0,0 +1,22 @@
+# Generated by Django 4.2.4 on 2023-10-30 11:50
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("pretixbase", "0248_item_free_price_suggestion"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="item",
+            name="hidden_if_item_available",
+            field=models.ForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                to="pretixbase.item",
+            ),
+        ),
+    ]

src/pretix/base/models/event.py

@@ -857,6 +857,10 @@ class Event(EventMixin, LoggedModel):
                 v.item = i
                 v.save(force_insert=True)
 
+        for i in self.items.filter(hidden_if_item_available__isnull=False):
+            i.hidden_if_item_available = item_map[i.hidden_if_item_available_id]
+            i.save()
+
         for imv in ItemMetaValue.objects.filter(item__event=other):
             imv.pk = None
             imv.property = item_meta_properties_map[imv.property_id]

src/pretix/base/models/items.py

@@ -494,13 +494,24 @@ class Item(LoggedModel):
         'Quota',
         null=True, blank=True,
         on_delete=models.SET_NULL,
-        verbose_name=_("Only show after sellout of"),
+        verbose_name=pgettext_lazy("hidden_if_available_legacy", "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.")
     )
+    hidden_if_item_available = models.ForeignKey(
+        'Item',
+        null=True, blank=True,
+        on_delete=models.SET_NULL,
+        verbose_name=_("Only show after sellout of"),
+        help_text=_("If you select a product here, this product will only be shown when that product is "
+                    "sold out. If combined with the option to hide sold-out products, this allows you to "
+                    "swap out products for more expensive ones once the cheaper option is sold out. There might "
+                    "be a short period in which both products are visible while all tickets of the referenced "
+                    "product are reserved, but not yet sold.")
+    )
     require_voucher = models.BooleanField(
         verbose_name=_('This product can only be bought using a voucher.'),
         default=False,