Fix form submission error when a multiple dropdown field question is empty

CHANGELOG.md

@@ -5,10 +5,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
-## Unreleased
+## [UNRELEASED]
 
 ### Fixed
 
+- Fix form submission error when a multiple dropdown field question is left empty
 - Fix search criterion value on multiple dropdown fields being reset after a search, which emptied the criterion when sorting or paging results
 
 ## [1.24.7] - 2026-10-28

inc/questiontype.class.php

@@ -295,13 +295,14 @@ public function formatRawAnswer(mixed $answer, Question $question): string
             case 'date':
                 return (string) $answer;
             case 'dropdown':
-                $answer = $answer['items_id'];
-                if (is_string($answer) || is_numeric($answer)) {
-                    $answer = [$answer];
-                }
+                // An empty multiple dropdown is submitted as an empty string
+                $ids = array_filter(
+                    array_map(intval(...), (array) ($answer['items_id'] ?? [])),
+                    fn(int $id) => $id > 0,
+                );
 
                 $itemtype = PluginFieldsDropdown::getClassname($current_field->fields['name']);
-                return implode(', ', array_map(fn($opt_id) => $itemtype::getById($opt_id)?->fields['name'] ?? '', $answer));
+                return implode(', ', array_map(fn(int $opt_id) => $itemtype::getById($opt_id)?->fields['name'] ?? '', $ids));
             case 'yesno':
                 return $answer ? __('Yes') : __('No');
             case 'datetime':

tests/Units/FieldQuestionTypeTest.php

@@ -247,6 +247,48 @@ public function testFormatRawAnswerForMultipleGlpiItemDropdown(): void
         }
     }
 
+    public function testFormatRawAnswerForMultipleDropdown(): void
+    {
+        $this->login();
+
+        // Arrange: create a multiple plugin dropdown field
+        $this->fields['multiple_dropdown'] = $this->createField([
+            'label'                                     => 'Multiple dropdown',
+            'type'                                      => 'dropdown',
+            'multiple'                                  => 1,
+            'default_value'                             => [],
+            PluginFieldsContainer::getForeignKeyField() => $this->block->getID(),
+            'ranking'                                   => 2,
+            'is_active'                                 => 1,
+        ], ['default_value']);
+
+        $dropdown_class = PluginFieldsDropdown::getClassname($this->fields['multiple_dropdown']->fields['name']);
+        $value1 = $this->createItem($dropdown_class, ['name' => 'Value 1', 'entities_id' => 0]);
+        $value2 = $this->createItem($dropdown_class, ['name' => 'Value 2', 'entities_id' => 0]);
+
+        $builder = new FormBuilder("My form");
+        $builder->addQuestion(
+            "Multiple dropdown question",
+            PluginFieldsQuestionType::class,
+            extra_data: json_encode($this->getFieldExtraDataConfig('multiple_dropdown')),
+        );
+        $form = $this->createForm($builder);
+        $question = Question::getById($this->getQuestionId($form, "Multiple dropdown question"));
+        $question_type = new PluginFieldsQuestionType();
+
+        // Act & Assert: an empty selection is submitted as an empty string
+        $this->assertEquals('', $question_type->formatRawAnswer([
+            'itemtype' => $dropdown_class,
+            'items_id' => '',
+        ], $question));
+
+        // Act & Assert: selected values are displayed
+        $this->assertEquals('Value 1, Value 2', $question_type->formatRawAnswer([
+            'itemtype' => $dropdown_class,
+            'items_id' => [(string) $value1->getID(), (string) $value2->getID()],
+        ], $question));
+    }
+
     public function testFieldsQuestionEditorRenderingForMultipleDropdownWithStringKeyedDefaultValue(): void
     {
         $this->login();