Architecture-components-samples: RxRoom create a flowable which filtered the null result of a sql query.How can i get the query result?

[basion]

I need a data , i firstly query it from database ,if the database did not contain the data, than i get the data from network and store it in database.
I use room and RxJava to do this
Here is the google code to create a flowable in RxRoom.java:

@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    public static <T> Flowable<T> createFlowable(final RoomDatabase database,
            final String[] tableNames, final Callable<T> callable) {
        return createFlowable(database, tableNames).observeOn(sAppToolkitIOScheduler)
                .map(new Function<Object, Optional<T>>() {
                    @Override
                    public Optional<T> apply(@NonNull Object o) throws Exception {
                        T data = callable.call();
                        return new Optional<>(data);
                    }
                }).filter(new Predicate<Optional<T>>() {
                    @Override
                    public boolean test(@NonNull Optional<T> optional) throws Exception {
                        return optional.mValue != null;
                    }
                }).map(new Function<Optional<T>, T>() {
                    @Override
                    public T apply(@NonNull Optional<T> optional) throws Exception {
                        return optional.mValue;
                    }
                });
    }

So if a sql query results nothing,the flowable will filter the original data.
But sometime we need the original result
How can i do it?

[rantianhua]

I have the same problem with you! Sometime when I open an new view, then I emit a request to load data, anyway I hope there is a result can be returned to view so that I can change it, but because the null data is filtered, I will never get a response in view if database doesn't have any data and first network request is failed, It's unbearable！

[florina-muntenescu]

You can return a Flowable<List<T>> instead of Flowable<T>.
If there are no rows in the table to be returned by your query, then the Flowable will emit an empty list.

[rantianhua]

@florina-muntenescu Thanks, got it!

[basion]

thank you!

[IP696]

@florina-muntenescu I have this query:

@Query("SELECT SUM(count*amount) FROM basket_items")
Flowable<Long> totalSum();

How can I return List???

https://stackoverflow.com/questions/51036938/flowables-callback-not-triggered-when-i-delete-last-row-in-room-db