RxSample.java

public Observable<DiffUtil.DiffResult> getAllStoreProducts(final List<Product> oldData) {
        return mApi.getAllStores().flatMap(new Func1<List<Store>, Observable<Store>>() {
            @Override
            public Observable<Store> call(List<Store> stores) {
                return Observable.from(stores);
            }
        }).flatMap(new Func1<Store, Observable<List<Product>>>() {
            @Override
            public Observable<List<Product>> call(Store store) {
                return mApi.getStoreProducts(store.getStoreId());
            }
        }, new Func2<Store, List<Product>, List<Product>>() {

            @Override
            public List<Product> call(Store store, List<Product> products) {
                for (int i = 0; i < products.size(); i++) {
                    Product product = products.get(i);
                    product.setStore(store);
                    products.set(i, product);
                }
                return products;
            }
        }).flatMap(new Func1<List<Product>, Observable<DiffUtil.DiffResult>>() {
            @Override
            public Observable<DiffUtil.DiffResult> call(List<Product> products) {
                return Observable.just(DiffUtil.calculateDiff(new ProductListDiffCallback(oldData, products)));
            }
        });

    }