FIX Add polymorphic class on new records for use in forms.

src/Forms/GridField/GridFieldDetailForm_ItemRequest.php

@@ -18,6 +18,7 @@
 use SilverStripe\ORM\FieldType\DBHTMLText;
 use SilverStripe\ORM\HasManyList;
 use SilverStripe\ORM\ManyManyList;
+use SilverStripe\ORM\PolymorphicHasManyList;
 use SilverStripe\ORM\RelationList;
 use SilverStripe\ORM\SS_List;
 use SilverStripe\ORM\ValidationException;
@@ -201,6 +202,12 @@ public function ItemEditForm()
             $key = $list->getForeignKey();
             $id = $list->getForeignID();
             $this->record->$key = $id;
+            // If the list is polymorphic, add the foreign class as well.
+            if ($list instanceof PolymorphicHasManyList) {
+                $classKey = $list->getForeignClassKey();
+                $class = $list->getForeignClass();
+                $this->record->$classKey = $class;
+            }
         }
 
         if (!$this->record->canView()) {

tests/php/Forms/GridField/GridFieldDetailFormTest.php

@@ -119,6 +119,25 @@ public function testAddForm()
         $this->assertEquals($count + 1, $group->People()->Count());
     }
 
+    public function testAddFormWithPolymorphicHasOne()
+    {
+        $group = PolymorphicPeopleGroup::get()
+            ->filter('Name', 'My Group')
+            ->sort('Name')
+            ->First();
+        $gridField = $group->getCMSFields()->dataFieldByName('People');
+        $detailForm = $gridField->getConfig()->getComponentByType(GridFieldDetailForm::class);
+
+        $reflectionDetailForm = new ReflectionClass($detailForm);
+        $reflectionMethod = $reflectionDetailForm->getMethod('getItemRequestHandler');
+        $reflectionMethod->setAccessible(true);
+        $itemrequest = $reflectionMethod->invoke($detailForm, [$gridField, $record, new Controller()]);
+        $itemrequest->ItemEditForm();
+
+        $this->assertEquals(PolymorphicPeopleGroup::class, $record->PolymorphicGroupClass);
+        $this->assertEquals(PolymorphicPeopleGroup::class, $record->PolymorphicGroupID);
+    }
+
     public function testViewForm()
     {
         $this->logInWithPermission('ADMIN');

tests/php/Forms/GridField/GridFieldDetailFormTest.yml

@@ -16,6 +16,10 @@ SilverStripe\Forms\Tests\GridField\GridFieldDetailFormTest\PeopleGroup:
       - =>SilverStripe\Forms\Tests\GridField\GridFieldDetailFormTest\Person.joe
       - =>SilverStripe\Forms\Tests\GridField\GridFieldDetailFormTest\Person.jane
 
+SilverStripe\Forms\Tests\GridField\GridFieldDetailFormTest\PolymorphicPeopleGroup:
+  group:
+    Name: My Group
+
 SilverStripe\Forms\Tests\GridField\GridFieldDetailFormTest\Category:
   category1:
     Name: Category 1

tests/php/Forms/GridField/GridFieldDetailFormTest/Person.php

@@ -18,7 +18,8 @@ class Person extends DataObject implements TestOnly
     ];
 
     private static $has_one = [
-        'Group' => PeopleGroup::class
+        'Group' => PeopleGroup::class,
+        'PolymorphicGroup' => DataObject::class,
     ];
 
     private static $many_many = [

tests/php/Forms/GridField/GridFieldDetailFormTest/PolymorphicPeopleGroup.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace SilverStripe\Forms\Tests\GridField\GridFieldDetailFormTest;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\Forms\GridField\GridField;
+use SilverStripe\Forms\GridField\GridFieldConfig_RelationEditor;
+use SilverStripe\ORM\DataObject;
+
+class PeopleGroup extends DataObject implements TestOnly
+{
+    private static $table_name = 'GridFieldDetailFormTest_PolymorphicPeopleGroup';
+
+    private static $db = [
+        'Name' => 'Varchar'
+    ];
+
+    private static $has_many = [
+        'People' => Person::class
+    ];
+
+    private static $default_sort = '"Name"';
+
+    public function getCMSFields()
+    {
+        $fields = parent::getCMSFields();
+        $fields->replaceField(
+            'People',
+            GridField::create(
+                'People',
+                'People',
+                $this->People(),
+                GridFieldConfig_RelationEditor::create()
+            )
+        );
+        return $fields;
+    }
+}