fix: returns undefined

packages/repository-tests/src/crud/relations/acceptance/belongs-to.relation.acceptance.ts

@@ -93,7 +93,7 @@ export function belongsToRelationAcceptance(
       expect(result).to.deepEqual(shipment);
     });
 
-    it('returns an empty array if the source instance does not have the foreign key', async () => {
+    it('returns undefined if the source instance does not have the foreign key', async () => {
       await shipmentRepo.create({
         name: 'Tuesday morning shipment',
       });
@@ -102,7 +102,7 @@ export function belongsToRelationAcceptance(
         description: 'Order that is shipped Tuesday morning',
       });
       const result = await orderRepo.shipment(order.id);
-      expect(result).to.deepEqual([]);
+      expect(result).to.be.undefined();
     });
 
     it('throws EntityNotFound error when the related model does not exist', async () => {

packages/repository/src/relations/belongs-to/belongs-to-accessor.ts

@@ -53,9 +53,10 @@ export function createBelongsToAccessor<
     const primaryKey = meta.keyTo;
     const sourceModel = await sourceRepository.findById(sourceId);
     const foreignKeyValue = sourceModel[foreignKey as keyof Source];
-    // different dbs use different empty values
-    if (foreignKeyValue === undefined || foreignKeyValue === null) {
-      return ([] as unknown) as Target;
+    // workaround to check referential integrity.
+    // should be removed once the memory connector ref integrity is done
+    if (!foreignKeyValue) {
+      return (undefined as unknown) as Target;
     }
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     const constraint: any = {[primaryKey]: foreignKeyValue};