Select IN() query in firebase database android

Question

What I want is to filter users from "uId", like Sqlite

SELECT * FROM users WHERE uId IN ('OTky08PtlIYDE4KHRIzvZxyqIKp1' ,  'UiHUd9BhpybDAmTiGTbgg30THPN2')

Is there any way from which we can directly get this kind of filter over firebase database?

Answer 1

I had the exact similar problem retrieving users by IDs. I found that there was no way of making the IN query in Firebase.

David suggests in his video either denormalizing the database or making a query for every id.

I did not like denormalizing so I queried every user. If you're in to RxJava, here's what I did

@NonNull
private Observable<List<User>> userIdsToUsers(@NonNull final List<String> uids) {
    if (uids.isEmpty()) {
        return Observable.just(new ArrayList<>());
    }

    return Observable.combineLatest(userIdsToUserObservables(uids),
            new ListCastFunc<>(User.class));
}

@NonNull
private List<Observable<User>> userIdsToUserObservables(@NonNull final List<String> uids) {
    final List<Observable<User>> result = new ArrayList<>();
    for (final String uid : uids) {
        result.add(userIdToUserObservable(uid));
    }
    return result;
}

@NonNull
private Observable<User> userIdToUserObservable(@NonNull final String uid) {
    return RxFirebaseDatabase.observeValueEvent(getUsersReference().orderByKey().equalTo(uid))
            .flatMap(this::dataSnapshotToUserObservable);
}


@NonNull
private Observable<User> dataSnapshotToUserObservable(@NonNull final DataSnapshot ds) {
    return Observable.fromCallable(() -> dataSnapshotToUser(ds));
}

@Nullable
private static User dataSnapshotToUser(@NonNull DataSnapshot ds) {
    if (!ds.exists()) {
        return null;
    }

    return parsedObjHere;
}

private static final class ListCastFunc<T> implements FuncN<List<T>> {

    @NonNull
    private final Class<T> targetClass;

    ListCastFunc(@NonNull final Class<T> targetClass) {
        this.targetClass = targetClass;
    }

    @Override
    public List<T> call(final Object... args) {
        if (args == null || args.length == 0) {
            return new ArrayList<>();
        }
        final List<T> results = new ArrayList<>(args.length);
        for (final Object arg : args) {
            if (arg != null && targetClass.isAssignableFrom(arg.getClass())) {
                //noinspection unchecked
                results.add((T) arg);
            }
        }
        return results;
    }
}