RxAndroid Retrofit how to fetch 2nd list only when 1st list emits are completed? - retrofit2

Question

Take a simple example given 2 lists Categories and Products
I need to sync on remote server list of local categories and store the returned remote_id on category item,
after that start sync on remote server list of products and store foreach remote_id on product item.
All categories need to sync all before start sync products, because
when sync products i need category id given from server
Main goal is
loop for each item on "categories" list
make API call for each item
ONLY when ALL api call are completed (because i need to get all rempote_id before start to sync products), start same loop on "products" list
In my example use the onComplete are wrong as it get fired when all list items are fetched and NOT when all api calls are completed.
How to solve? What operator should use?
... from ApiService.java
// Create category
#FormUrlEncoded
#POST("Category/")
Single<ApiResponse> createCategory(#Field("Description") String description, #Field("ShortDescription") String shortDescription);
#FormUrlEncoded
#POST("Product/")
Single<ApiResponse> createProduct(#Field("ProductCode") String ProductCode,
#Field("Description") String Description,
#Field("ShortDescription") String ShortDescription,
#Field("Quantity") int Quantity,
#Field("MeasurementUnit_ID") int MeasurementUnit_ID,
#Field("SellPrice") int SellPrice,
#Field("Category_ID") int Category_ID,
#Field("Vat_ID") int Vat_ID
);
....
// MainActivity
List<Category> cats = new CategoryOperations(this).getUnsynced();
List<Product> prods = new ProductOperations(this).getUnsynced();
Observable.fromIterable(cats)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Observer<Category>() {
#Override
public void onSubscribe(Disposable d) {
}
#Override
public void onNext(Category c) {
apiService.createCategory(c.getDescription(), c.getDescription())
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribeWith(new DisposableSingleObserver<ApiResponse>() {
#Override
public void onSuccess(ApiResponse resp) {
if (resp.id > 0 && resp.success) {
//Toast.makeText(getApplicationContext(), "Created category id: " + resp.id, Toast.LENGTH_LONG).show();
Log.d(TAG, "Created category id: " + resp.id );
c.setCloudId(resp.id);
new CategoryOperations(ctx).update(c);
return;
}else{
Log.d(TAG, "Category not created " + resp.message);
}
}
#Override
public void onError(Throwable e) {
Log.e(TAG, "onError: " + e.getMessage());
}
}
);
}
#Override
public void onError(Throwable e) {
}
#Override
public void onComplete() {
Log.d(TAG, "All categories synced");
Log.d(TAG, "Now can start to sync products");
Log.d(TAG, "Obvioussly error here as this complete only fired when all items are fetched and NOT");
Log.d(TAG, "When all api calls are completed");
}
});