Swap the variations resource controller method

src/Routing/JsonapiRouteSubscriber.php

+<?php declare(strict_types=1);
+
+namespace Drupal\commerce_api\Routing;
+
+use Drupal\Core\Routing\RouteSubscriberBase;
+use Drupal\jsonapi\ResourceType\ResourceType;
+use Drupal\jsonapi\ResourceType\ResourceTypeRepositoryInterface;
+use Drupal\jsonapi\Routing\Routes as JsonapiRoutes;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\Routing\RouteCollection;
+
+final class JsonapiRouteSubscriber extends RouteSubscriberBase {
+
+  /**
+   * The resource type repository.
+   *
+   * @var \Drupal\jsonapi\ResourceType\ResourceTypeRepositoryInterface
+   */
+  private $resourceTypeRepository;
+
+  /**
+   * Constructs a new JsonapiRouteSubscriber object.
+   *
+   * @param \Drupal\jsonapi\ResourceType\ResourceTypeRepositoryInterface $resource_type_repository
+   *   The resource type repository.
+   */
+  public function __construct(ResourceTypeRepositoryInterface $resource_type_repository) {
+    $this->resourceTypeRepository = $resource_type_repository;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function alterRoutes(RouteCollection $collection) {
+    $product_resource_types = array_filter($this->resourceTypeRepository->all(), static function (ResourceType $resource_type) {
+      return $resource_type->getEntityTypeId() === 'commerce_product';
+    });
+    foreach ($product_resource_types as $resource_type) {
+      $variations_relationship_route_name = JsonapiRoutes::getRouteName($resource_type, 'variations.relationship.get');
+      $variations_relationship_route = $collection->get($variations_relationship_route_name);
+      if ($variations_relationship_route !== NULL) {
+        $variations_relationship_route->setDefault(RouteObjectInterface::CONTROLLER_NAME, 'commerce_api.jsonapi_controller_shim:getVariationRelationship');
+      }
+    }
+  }
+
+}