fix(core): Guard relation custom field resolution against a missing entity id

packages/core/e2e/custom-field-relations.e2e-spec.ts

@@ -454,6 +454,51 @@ describe('Custom field relations', () => {
 
                 assertCustomFieldIds(updateCustomerAddress.customFields, 'T_3', ['T_2', 'T_4']);
             });
+
+            // Regression: an OrderAddress (Order.shippingAddress / billingAddress) is an embedded
+            // value object with no `id`, so its relation custom fields cannot be resolved by a parent
+            // entity id. Previously this threw "The loader.load() function must be called with a
+            // value, but got: undefined"; it must now resolve to null / [] instead of crashing.
+            it('order shippingAddress resolves relation custom fields without throwing', async () => {
+                await shopClient.asUserWithCredentials('hayden.zieme12@hotmail.com', 'test');
+                await shopClient.query(gql`
+                    mutation {
+                        addItemToOrder(productVariantId: "T_1", quantity: 1) {
+                            ... on Order {
+                                id
+                            }
+                        }
+                    }
+                `);
+                await shopClient.query(gql`
+                    mutation {
+                        setOrderShippingAddress(
+                            input: {
+                                countryCode: "GB"
+                                streetLine1: "Test Street"
+                                customFields: { singleId: "T_1", multiIds: ["T_1", "T_2"] }
+                            }
+                        ) {
+                            ... on Order {
+                                id
+                            }
+                        }
+                    }
+                `);
+
+                const { activeOrder } = await shopClient.query(gql`
+                    query {
+                        activeOrder {
+                            shippingAddress {
+                                ${customFieldsSelection}
+                            }
+                        }
+                    }
+                `);
+
+                expect(activeOrder.shippingAddress.customFields.single).toBeNull();
+                expect(activeOrder.shippingAddress.customFields.multi).toEqual([]);
+            });
         });
 
         describe('Collection entity', () => {

packages/core/src/api/common/custom-field-relation-resolver.service.ts

@@ -16,7 +16,7 @@ import { RequestContext } from './request-context';
 
 export interface ResolveRelationConfig {
     ctx: RequestContext;
-    entityId: ID;
+    entityId: ID | undefined;
     entityName: string;
     fieldDef: RelationCustomFieldConfig;
 }
@@ -40,6 +40,15 @@ export class CustomFieldRelationResolverService {
     async resolveRelation(config: ResolveRelationConfig): Promise<VendureEntity | VendureEntity[] | null> {
         const { ctx, entityId, entityName, fieldDef } = config;
 
+        // Embedded value objects such as OrderAddress (Order.shippingAddress / billingAddress) have
+        // no `id`, so `entityId` can be undefined. Passing it to DataLoader.load() would throw
+        // ("The loader.load() function must be called with a value, but got: undefined"); there is
+        // nothing to resolve by id, so return an empty result, matching the behaviour before
+        // relation custom fields were resolved via DataLoader.
+        if (entityId == null) {
+            return fieldDef.list ? [] : null;
+        }
+
         const loader = this.getLoader(ctx, entityName, fieldDef);
         const batchResult = await loader.load(entityId);
         const finalResult = fieldDef.list ? batchResult : (batchResult[0] ?? null);