Prevent false deprecation error for n to m associations

src/Controller/CRUDController.php

@@ -1554,7 +1554,13 @@ private function checkParentChildAssociation(Request $request, object $object):
         $propertyAccessor = PropertyAccess::createPropertyAccessor();
         $propertyPath = new PropertyPath($this->admin->getParentAssociationMapping());
 
-        if ($parentAdmin->getObject($parentId) !== $propertyAccessor->getValue($object, $propertyPath)) {
+        $parentObject = $parentAdmin->getObject($parentId);
+        $objectParent = $propertyAccessor->getValue($object, $propertyPath);
+
+        // $objectParent may be an array or a Collection when the parent association is many to many.
+        $parentObjectMatches = self::equalsOrInList($parentObject, $objectParent);
+
+        if (!$parentObjectMatches) {
             // NEXT_MAJOR: make this exception
             @trigger_error(
                 'Accessing a child that isn\'t connected to a given parent is deprecated since sonata-project/admin-bundle 3.34 and won\'t be allowed in 4.0.',
@@ -1563,6 +1569,31 @@ private function checkParentChildAssociation(Request $request, object $object):
         }
     }
 
+    /**
+     * Checks whether $parentObject is equal to $childParentValue or part of it.
+     *
+     * @param object          $needle   Object to compare with $haystack
+     * @param object|iterable $haystack Object to compare with $needle
+     *
+     * @return bool true when $haystack equals $needle or $haystack is iterable and contains $needle
+     */
+    private static function equalsOrInList(object $needle, $haystack): bool
+    {
+        if ($needle === $haystack) {
+            return true;
+        }
+
+        if (is_iterable($haystack)) {
+            foreach ($haystack as $haystackItem) {
+                if ($haystackItem === $needle) {
+                    return true;
+                }
+            }
+        }
+
+        return false;
+    }
+
     /**
      * Sets the admin form theme to form view. Used for compatibility between Symfony versions.
      */

tests/Controller/CRUDControllerTest.php

@@ -1361,6 +1361,42 @@ public function testDeleteActionInvalidCsrfToken(): void
         }
     }
 
+    public function testDeleteActionChildManyToMany(): void
+    {
+        $parent = new \stdClass();
+
+        $child = new \stdClass();
+        $child->parents = [$parent];
+
+        $parentAdmin = $this->createMock(PostAdmin::class);
+        $parentAdmin->method('getIdParameter')->willReturn('parent_id');
+
+        $childAdmin = $this->admin;
+        $childAdmin->method('getIdParameter')->willReturn('parent_id');
+
+        $parentAdmin->expects($this->once())
+            ->method('getObject')
+            ->willReturn($parent);
+
+        $childAdmin->expects($this->once())
+            ->method('getObject')
+            ->willReturn($child);
+
+        $childAdmin->expects($this->once())
+            ->method('isChild')
+            ->willReturn(true);
+
+        $childAdmin->expects($this->once())
+            ->method('getParent')
+            ->willReturn($parentAdmin);
+
+        $childAdmin->expects($this->exactly(2))
+            ->method('getParentAssociationMapping')
+            ->willReturn('parents');
+
+        $this->controller->deleteAction(1);
+    }
+
     public function testEditActionNotFoundException(): void
     {
         $this->expectException(NotFoundHttpException::class);