How to make HATEOAS render empty embedded array

Question

Usually CollectionModel will return an _embedded array, but in this example:

@GetMapping("/{id}/productMaterials")
    public ResponseEntity<?> getProductMaterials(@PathVariable Integer id) {
        Optional<Material> optionalMaterial = materialRepository.findById(id);
        if (optionalMaterial.isPresent()) {
            List<ProductMaterial> productMaterials = optionalMaterial.get().getProductMaterials();
            CollectionModel<ProductMaterialModel> productMaterialModels =
                    new ProductMaterialModelAssembler(ProductMaterialController.class, ProductMaterialModel.class).
                            toCollectionModel(productMaterials);
            return ResponseEntity.ok().body(productMaterialModels);
        }
        return ResponseEntity.badRequest().body("no such material");
    }

if the productMaterials is empty CollectionModel will not render the _embedded array which will break the client. Is there any ways to fix this?

you will have to use an EmbeddedWrapper stackoverflow.com/questions/30286795/… you should mark you resource explicitly to render an empty _embedded array — Amr Alaa
– Amr Alaa, Commented Nov 1, 2020 at 8:44
Thank you for your answer. Could you show me how to write it? I have no idea what the author was talking about :(( — La Hai
– La Hai, Commented Nov 1, 2020 at 9:03

Amr Alaa · Accepted Answer · 2020-11-01 09:20:40Z

3

if (optionalMaterial.isPresent()) {
        List<ProductMaterial> productMaterials = optionalMaterial.get().getProductMaterials();
        CollectionModel<ProductMaterialModel> productMaterialModels =
                new ProductMaterialModelAssembler(ProductMaterialController.class, ProductMaterialModel.class).
                        toCollectionModel(productMaterials);
        if(productMaterialModels.isEmpty()) {
            EmbeddedWrappers wrappers = new EmbeddedWrappers(false);
            EmbeddedWrapper wrapper = wrappers.emptyCollectionOf(ProductMaterialModel.class);
            Resources<Object> resources = new Resources<>(Arrays.asList(wrapper));
            return ResponseEntity.ok(new Resources<>(resources));
        } else {
            return ResponseEntity.ok().body(productMaterialModels);
        }
    }

answered Nov 1, 2020 at 9:20

Amr Alaa

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4 Comments

La Hai Over a year ago

Thank you. it worked :)) since the Resources does not exist anymore, i change the code a little bit

La Hai Over a year ago

Is spring HATEOAS popular right now? All the solutions seem to be deprecated and not elegant at all.

Raman Over a year ago

@LaHai how did you change the code? I can't find the Resources class.

La Hai Over a year ago

@Raman Happy new year. I believe it's CollectionModel now. You can find the changes in here

jalogar · Accepted Answer · 2023-10-18 10:38:26Z

0

It could be easily done now using this code:

    List<ProductMaterial> productMaterials = optionalMaterial.get().getProductMaterials();
    return HalModelBuilder.emptyHalModel()
            .embed(productMaterials, ProductMaterial.class)
            .build();

answered Oct 18, 2023 at 10:38

jalogar

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