Allow dependencies between questions (#1202)

- [x] data model
- [x] api
- [x] backend editor
- [x] backend validation logic
- [x] frontend display logic
- [x] frontend validation logic
- [x] test checkout step
- [x] test modify order in frontend
- [x] test modify order in backend
- [x] validation tests
- [x] correctly evaluate dependency tree in frontend?
- [x] copy events

src/pretix/api/serializers/item.py

@@ -159,12 +159,20 @@ class QuestionSerializer(I18nAwareModelSerializer):
     class Meta:
         model = Question
         fields = ('id', 'question', 'type', 'required', 'items', 'options', 'position',
-                  'ask_during_checkin', 'identifier')
+                  'ask_during_checkin', 'identifier', 'dependency_question', 'dependency_value')
 
     def validate_identifier(self, value):
         Question._clean_identifier(self.context['event'], value, self.instance)
         return value
 
+    def validate_dependency_question(self, value):
+        if value:
+            if value.type not in (Question.TYPE_CHOICE, Question.TYPE_BOOLEAN, Question.TYPE_CHOICE_MULTIPLE):
+                raise ValidationError('Question dependencies can only be set to boolean or choice questions.')
+        if value == self.instance:
+            raise ValidationError('A question cannot depend on itself.')
+        return value
+
     def validate(self, data):
         data = super().validate(data)
         if self.instance and 'options' in data:
@@ -176,6 +184,18 @@ class QuestionSerializer(I18nAwareModelSerializer):
         full_data = self.to_internal_value(self.to_representation(self.instance)) if self.instance else {}
         full_data.update(data)
 
+        if full_data.get('ask_during_checkin') and full_data.get('dependency_question'):
+            raise ValidationError('Dependencies are not supported during check-in.')
+
+        dep = full_data.get('dependency_question')
+        if dep:
+            seen_ids = {self.instance.pk} if self.instance else set()
+            while dep:
+                if dep.pk in seen_ids:
+                    raise ValidationError(_('Circular dependency between questions detected.'))
+                seen_ids.add(dep.pk)
+                dep = dep.dependency_question
+
         Question.clean_items(event, full_data.get('items'))
         return data