QuestionAnswer: Add UNIQUE keys on (orderposition, question) and (cartposition, question) (#1994)

src/pretix/base/migrations/0179_auto_20210311_1653.py

@@ -0,0 +1,49 @@
+# Generated by Django 3.0.10 on 2021-03-11 16:53
+
+from django.db import migrations
+
+
+def clean_duplicates(apps, schema_editor):
+    while True:
+        delete_options = """
+            DELETE
+            FROM pretixbase_questionanswer_options
+            WHERE questionanswer_id IN (
+                SELECT MIN(qa.id)
+                FROM pretixbase_questionanswer qa
+                GROUP BY qa.cartposition_id, qa.orderposition_id, qa.question_id
+                HAVING COUNT(*) > 1
+            );
+        """
+        delete_answers = """
+            DELETE
+            FROM pretixbase_questionanswer
+            WHERE pretixbase_questionanswer.id IN (
+                SELECT MIN(qa.id)
+                FROM pretixbase_questionanswer qa
+                GROUP BY qa.cartposition_id, qa.orderposition_id, qa.question_id
+                HAVING COUNT(*) > 1
+            );
+        """
+        with schema_editor.connection.cursor() as cursor:
+            cursor.execute(delete_options)
+            cursor.execute(delete_answers)
+            if cursor.rowcount == 0:
+                return
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0178_auto_20210308_1326'),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            clean_duplicates,
+            migrations.RunPython.noop,
+        ),
+        migrations.AlterUniqueTogether(
+            name='questionanswer',
+            unique_together={('orderposition', 'question'), ('cartposition', 'question')},
+        ),
+    ]

src/pretix/base/models/orders.py

@@ -983,6 +983,9 @@ class QuestionAnswer(models.Model):
 
     objects = ScopedManager(organizer='question__event__organizer')
 
+    class Meta:
+        unique_together = [['orderposition', 'question'], ['cartposition', 'question']]
+
     @property
     def backend_file_url(self):
         if self.file: