fix(repository): belongsto accessor should return undefined if foreig…

…n key is not included

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

@@ -93,6 +93,18 @@ export function belongsToRelationAcceptance(
       expect(result).to.deepEqual(shipment);
     });
 
+    it('returns undefined if the source instance does not have the foreign key', async () => {
+      await shipmentRepo.create({
+        name: 'Tuesday morning shipment',
+      });
+      const order = await orderRepo.create({
+        // doesn't have the foreign key
+        description: 'Order that is shipped Tuesday morning',
+      });
+      const result = await orderRepo.shipment(order.id);
+      expect(result).to.be.undefined();
+    });
+
     it('throws EntityNotFound error when the related model does not exist', async () => {
       const deletedCustomer = await customerRepo.create({
         name: 'Order McForder',

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

@@ -53,6 +53,12 @@ export function createBelongsToAccessor<
     const primaryKey = meta.keyTo;
     const sourceModel = await sourceRepository.findById(sourceId);
     const foreignKeyValue = sourceModel[foreignKey as keyof Source];
+    // workaround to check referential integrity.
+    // should be removed once the memory connector ref integrity is done
+    // GH issue: https://github.com/strongloop/loopback-next/issues/2333
+    if (!foreignKeyValue) {
+      return (undefined as unknown) as Target;
+    }
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     const constraint: any = {[primaryKey]: foreignKeyValue};
     const constrainedRepo = new DefaultBelongsToRepository(