Improve validation

src/pretix/api/serializers/item.py

@@ -668,6 +668,7 @@ class InlineQuestionnaireChildSerializer(I18nAwareModelSerializer):
 
 
 class QuestionnaireSerializer(I18nAwareModelSerializer):
+    items = serializers.PrimaryKeyRelatedField(many=True, queryset=Item.objects.none(), allow_null=True, required=False)
     limit_sales_channels = serializers.SlugRelatedField(
         slug_field="identifier",
         queryset=SalesChannel.objects.none(),
@@ -682,6 +683,7 @@ class QuestionnaireSerializer(I18nAwareModelSerializer):
 
     def __init__(self, *args, **kwargs):
         self.fields['children'] = InlineQuestionnaireChildSerializer(many=True, required=True, context=kwargs['context'], partial=False)
+        self.fields['items'].child_relation.queryset = kwargs['context']['event'].items.all()
         self.fields['limit_sales_channels'].child_relation.queryset = kwargs['context']['event'].organizer.sales_channels.all()
         super().__init__(*args, **kwargs)
 
@@ -725,6 +727,10 @@ class QuestionnaireSerializer(I18nAwareModelSerializer):
         children_data = validated_data.pop('children') if 'children' in validated_data else []
         questionnaire = super().create(validated_data)
         self.set_children(questionnaire, children_data)
+        try:
+            questionnaire.check_constraints()
+        except ValidationError as e:
+            raise DrfValidationError(as_serializer_error(e))
         return questionnaire
 
     @transaction.atomic
@@ -733,6 +739,10 @@ class QuestionnaireSerializer(I18nAwareModelSerializer):
         questionnaire = super().update(instance, validated_data)
         if children_data is not None:
             self.set_children(questionnaire, children_data)
+        try:
+            questionnaire.check_constraints()
+        except ValidationError as e:
+            raise DrfValidationError(as_serializer_error(e))
         return questionnaire
 
     def set_children(self, questionnaire, new_data):

src/pretix/base/models/items.py

@@ -35,6 +35,8 @@
 
 import calendar
 import os
+from itertools import groupby
+
 import sys
 import uuid
 import warnings
@@ -2040,6 +2042,41 @@ class Questionnaire(LoggedModel):
     class Meta:
         ordering = ('position', 'id')
 
+    def __str__(self) -> str:
+        return self.internal_name
+
+    @property
+    def sortkey(self):
+        return self.position, self.id
+
+    def __lt__(self, other) -> bool:
+        return self.sortkey < other.sortkey
+
+    def check_constraints(self):
+        errors = set()
+        all_questionnaires = Questionnaire.objects.filter(event=self.event).prefetch_related('children', 'items')
+        for (type, item_id, sdf, udf), fields in groupby(sorted(
+            (q.type, item.id, c.system_datafield or '', c.user_datafield_id or 0, q, item)
+            for q in all_questionnaires
+            for item in q.items.all()
+            for c in q.children.all()
+        ), key=lambda d: (d[0], d[1], d[2], d[3])):
+            fields = list(fields)
+            if len(fields) > 1:
+                type, item_id, sdf, udf, q, item = fields[0]
+                questionnaires = set(q for type,item,sdf,udf,q,item in fields)
+                df_label = _('System data field') + f' "{sdf}"' if sdf else _('User-defined data field') + f' "{str(udf)}"'
+                if len(questionnaires) == 1:
+                    errors.add(_('{datafield} added twice to questionnaire "{questionnaire}"').format(
+                        datafield=df_label, questionnaire=next(str(q) for q in questionnaires)
+                    ))
+                else:
+                    errors.add(_('{datafield} added twice to the same item "{item}" via questionnaires "{questionnaires}"').format(
+                        datafield=df_label, item=str(item), questionnaires='" and "'.join(str(q) for q in questionnaires)
+                    ))
+        if errors:
+            raise ValidationError(list(errors))
+
 
 class QuestionnaireChild(LoggedModel):
     class SystemQuestion(models.TextChoices):

src/pretix/static/pretixcontrol/js/ui/questionnaires/QuestionnaireElement.vue

@@ -42,7 +42,7 @@ function addExistingDatafield (field) {
 		_cid: useId(),
 		question: field.id,
 		required: false,
-		label: field.question,
+		label: field.question ?? {en: field.internal_name},
 		help_text: {},
 		dependency_question: null,
 		dependency_values: [],
@@ -112,7 +112,7 @@ const isEditable = computed(() => props.selected_product && props.questionnaire.
     <summary class="questionnaire-panel-heading">
 			<div class=" editor-row">
 				<div class="editor-preview-area">
-					<input type="checkbox" @change="e => {setListState(props.questionnaire.items, selected_product, e.target.checked); emit('update')}" v-if="selected_product && !preview_mode" :checked="!isHidden">
+					<input type="checkbox" @change="e => {setListState(props.questionnaire.items, e.target.checked, selected_product); emit('update')}" v-if="selected_product && !preview_mode" :checked="!isHidden">
 					{{ props.questionnaire.internal_name }}
 					<span class="fa fa-warning" v-if="questionnaire._err_mes"></span>
 					<span class="fa fa-cog fa-spin" v-if="questionnaire._loading"></span>
@@ -125,7 +125,15 @@ const isEditable = computed(() => props.selected_product && props.questionnaire.
 			</div>
     </summary>
     <div class="questionnaire-panel-body" v-if="!isHidden">
-			<div class="alert alert-warning" v-if="questionnaire._err_mes">{{ questionnaire._err_mes }}</div>
+			<div class="alert alert-warning" v-if="questionnaire._err_mes">
+        <dl v-if="questionnaire._err_mes.api_error">
+          <template v-for="(messages, key) in questionnaire._err_mes.api_error">
+            <dt>{{ key }}</dt>
+            <dd><ul><li v-for="message in messages">{{ message }}</li></ul></dd>
+          </template>
+        </dl>
+        <p v-else>{{ questionnaire._err_mes }}</p>
+      </div>
       <div class="form-horizontal" :id="`questionListParent${props.questionnaire.id}`">
 				<SlickList axis="y" v-model:list="props.questionnaire.children" useDragHandle :appendTo="`#questionListParent${props.questionnaire.id}`" @update:list="emit('update')">
 					<SlickItem v-for="(child, index) in props.questionnaire.children" :key="child._cid" :index="index">
@@ -167,7 +175,7 @@ const isEditable = computed(() => props.selected_product && props.questionnaire.
                   :title="gettext('Add existing data field')">
 
 				<div class="list-group">
-					<a href="javascript:" @click="addExistingDatafield(field)" v-for="field in datafields" class="list-group-item">{{ i18n_any(field.question) }}</a>
+					<a href="javascript:" @click="addExistingDatafield(field)" v-for="field in datafields" class="list-group-item">{{ field.internal_name ?? i18n_any(field.question) }}</a>
 				</div>
 
         <button @click="dlgAddExisting.close()" class="btn btn-default pull-right">{{ gettext('Cancel') }}</button>