FIX many_many through should allow subclasses (#11230)

```php
class HomePage extends Page
{
    private static $many_many = [
        'HeroImages' => [
            'through' => PageImageLink::class,
            'from' => 'Page',
            'to' => 'Image',
        ]
    ];

}
```

```php
class PageImageLink extends DataObject
{
    private static $has_one = [
        'Page' => SiteTree::class,
        'Image' => Image::class,
    ];
}

This fails because the linking object's relation class doesn't exactly match the owner. Sharing the linking objects across various entries in the ancestry should be a supported use case.

Co-authored-by: Aaron Carlino <unclecheese@leftandmain.com>

src/ORM/DataObjectSchema.php

@@ -1239,7 +1239,7 @@ protected function checkManyManyFieldClass($parentClass, $component, $joinClass,
         }
 
         // Validate bad types on parent relation
-        if ($key === 'from' && $relationClass !== $parentClass) {
+        if ($key === 'from' && $relationClass !== $parentClass && !is_subclass_of($parentClass, $relationClass)) {
             throw new InvalidArgumentException(
                 "many_many through relation {$parentClass}.{$component} {$key} references a field name "
                 . "{$joinClass}::{$relation} of type {$relationClass}; {$parentClass} expected"

tests/php/ORM/ManyManyThroughListTest.php

@@ -25,10 +25,12 @@ class ManyManyThroughListTest extends SapphireTest
         ManyManyThroughListTest\Item::class,
         ManyManyThroughListTest\JoinObject::class,
         ManyManyThroughListTest\TestObject::class,
+        ManyManyThroughListTest\TestObjectSubclass::class,
         ManyManyThroughListTest\PolyItem::class,
         ManyManyThroughListTest\PolyJoinObject::class,
         ManyManyThroughListTest\PolyObjectA::class,
         ManyManyThroughListTest\PolyObjectB::class,
+        ManyManyThroughListTest\PseudoPolyJoinObject::class,
         ManyManyThroughListTest\Locale::class,
         ManyManyThroughListTest\FallbackLocale::class,
     ];
@@ -161,46 +163,82 @@ public function sortingProvider()
         ];
     }
 
-    public function testAdd()
+    public function provideAdd(): array
     {
-        /** @var ManyManyThroughListTest\TestObject $parent */
-        $parent = $this->objFromFixture(ManyManyThroughListTest\TestObject::class, 'parent1');
+        return [
+            [
+                'parentClass' => ManyManyThroughListTest\TestObject::class,
+                'joinClass' => ManyManyThroughListTest\JoinObject::class,
+                'joinProperty' => 'ManyManyThroughListTest_JoinObject',
+                'relation' => 'Items',
+            ],
+            [
+                'parentClass' => ManyManyThroughListTest\TestObjectSubclass::class,
+                'joinClass' => ManyManyThroughListTest\PseudoPolyJoinObject::class,
+                'joinProperty' => 'ManyManyThroughListTest_PseudoPolyJoinObject',
+                'relation' => 'MoreItems',
+            ],
+        ];
+    }
+
+    /**
+     * @dataProvider provideAdd
+     */
+    public function testAdd(string $parentClass, string $joinClass, string $joinProperty, string $relation)
+    {
+        $parent = $this->objFromFixture($parentClass, 'parent1');
         $newItem = new ManyManyThroughListTest\Item();
         $newItem->Title = 'my new item';
         $newItem->write();
-        $parent->Items()->add($newItem, ['Title' => 'new join record']);
+        $parent->$relation()->add($newItem, ['Title' => 'new join record']);
 
         // Check select
-        $newItem = $parent->Items()->filter(['Title' => 'my new item'])->first();
+        $newItem = $parent->$relation()->filter(['Title' => 'my new item'])->first();
         $this->assertNotNull($newItem);
         $this->assertEquals('my new item', $newItem->Title);
         $this->assertInstanceOf(
-            ManyManyThroughListTest\JoinObject::class,
+            $joinClass,
             $newItem->getJoin()
         );
         $this->assertInstanceOf(
-            ManyManyThroughListTest\JoinObject::class,
-            $newItem->ManyManyThroughListTest_JoinObject
+            $joinClass,
+            $newItem->$joinProperty
         );
-        $this->assertEquals('new join record', $newItem->ManyManyThroughListTest_JoinObject->Title);
+        $this->assertEquals('new join record', $newItem->$joinProperty->Title);
     }
 
-    public function testRemove()
+    public function provideRemove(): array
     {
-        /** @var ManyManyThroughListTest\TestObject $parent */
-        $parent = $this->objFromFixture(ManyManyThroughListTest\TestObject::class, 'parent1');
+        return [
+            [
+                'parentClass' => ManyManyThroughListTest\TestObject::class,
+                'relation' => 'Items',
+            ],
+            [
+                'parentClass' => ManyManyThroughListTest\TestObjectSubclass::class,
+                'relation' => 'MoreItems',
+            ],
+        ];
+    }
+
+    /**
+     * @dataProvider provideRemove
+     */
+    public function testRemove(string $parentClass, string $relation)
+    {
+        $parent = $this->objFromFixture($parentClass, 'parent1');
         $this->assertListEquals(
             [
                 ['Title' => 'item 1'],
                 ['Title' => 'item 2']
             ],
-            $parent->Items()
+            $parent->$relation()
         );
-        $item1 = $parent->Items()->filter(['Title' => 'item 1'])->first();
-        $parent->Items()->remove($item1);
+        $item1 = $parent->$relation()->filter(['Title' => 'item 1'])->first();
+        $parent->$relation()->remove($item1);
         $this->assertListEquals(
             [['Title' => 'item 2']],
-            $parent->Items()
+            $parent->$relation()
         );
     }
 

tests/php/ORM/ManyManyThroughListTest.yml

@@ -3,6 +3,11 @@ SilverStripe\ORM\Tests\ManyManyThroughListTest\TestObject:
     Title: 'my object'
   parent2:
     Title: 'my object2'
+SilverStripe\ORM\Tests\ManyManyThroughListTest\TestObjectSubclass:
+  parent1:
+    Title: 'my object'
+  parent2:
+    Title: 'my object2'
 SilverStripe\ORM\Tests\ManyManyThroughListTest\Item:
   # Having this one first means the IDs of records aren't the same as the IDs of the join objects.
   child0:
@@ -30,6 +35,25 @@ SilverStripe\ORM\Tests\ManyManyThroughListTest\JoinObject:
     Title: 'join 4'
     Parent: =>SilverStripe\ORM\Tests\ManyManyThroughListTest\TestObject.parent2
     Child: =>SilverStripe\ORM\Tests\ManyManyThroughListTest\Item.child2
+SilverStripe\ORM\Tests\ManyManyThroughListTest\PseudoPolyJoinObject:
+  join1:
+    Title: 'join 1'
+    Sort: 4
+    Parent: =>SilverStripe\ORM\Tests\ManyManyThroughListTest\TestObjectSubclass.parent1
+    Child: =>SilverStripe\ORM\Tests\ManyManyThroughListTest\Item.child1
+  join2:
+    Title: 'join 2'
+    Sort: 2
+    Parent: =>SilverStripe\ORM\Tests\ManyManyThroughListTest\TestObjectSubclass.parent1
+    Child: =>SilverStripe\ORM\Tests\ManyManyThroughListTest\Item.child2
+  join3:
+    Title: 'join 3'
+    Parent: =>SilverStripe\ORM\Tests\ManyManyThroughListTest\TestObjectSubclass.parent2
+    Child: =>SilverStripe\ORM\Tests\ManyManyThroughListTest\Item.child1
+  join4:
+    Title: 'join 4'
+    Parent: =>SilverStripe\ORM\Tests\ManyManyThroughListTest\TestObjectSubclass.parent2
+    Child: =>SilverStripe\ORM\Tests\ManyManyThroughListTest\Item.child2
 SilverStripe\ORM\Tests\ManyManyThroughListTest\PolyObjectA:
   obja1:
     Title: 'object A1'

tests/php/ORM/ManyManyThroughListTest/PseudoPolyJoinObject.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace SilverStripe\ORM\Tests\ManyManyThroughListTest;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+
+/**
+ * @property string $Title
+ * @method TestObject Parent()
+ * @method Item Child()
+ */
+class PseudoPolyJoinObject extends DataObject implements TestOnly
+{
+    private static $table_name = 'ManyManyThroughListTest_PseudoPolyJoinObject';
+
+    private static $db = [
+        'Title' => 'Varchar',
+        'Sort' => 'Int',
+    ];
+
+    private static $has_one = [
+        'Parent' => TestObject::class,
+        'Child' => Item::class,
+    ];
+
+    private static $default_sort = '"Sort" ASC';
+}

tests/php/ORM/ManyManyThroughListTest/TestObjectSubclass.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace SilverStripe\ORM\Tests\ManyManyThroughListTest;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+use SilverStripe\ORM\ManyManyThroughList;
+
+/**
+ * Basic parent object
+ *
+ * @property string $Title
+ * @method ManyManyThroughList Items()
+ */
+class TestObjectSubclass extends TestObject implements TestOnly
+{
+    private static $table_name = 'ManyManyThroughListTest_TestObjectSubclass';
+
+    private static $db = [
+        'Title' => 'Varchar'
+    ];
+
+    private static $many_many = [
+        'MoreItems' => [
+            'through' => PseudoPolyJoinObject::class,
+            'from' => 'Parent',
+            'to' => 'Child',
+        ]
+    ];
+}