Develop Reference

Size of an array of a Room Query

in my Dao I've defined a Query like this to check whether the database is empty or not:
#Query("SELECT * from meal_table LIMIT 1")
suspend fun getAnyMeal(): LiveData<Array<Meal>>
Within my populateDatabse function I would like to check, whether any item is inside my database with something like this:
suspend fun populateDatabase(mealDao: MealDao) {
if ((mealDao.getAnyMeal()).size < 1)
...
}
Unforunately size doesnt work in this context unless I am doing something wrong.
If someone has a tipp on how to solve this I would apreciate it! Thank you!

Unforunately size doesnt work in this context
It is because getAnyMeal returns you LiveData which has no property named size. LiveData is an observable object. It means that LiveData object you get by calling this method will return to its observers (only the ones who "subscribed" to updates) an array of Meal objects when this array will be available.
First of all, when you are using LiveData with Room you sort of giving Room a signal that you are not requesting a response immediately. LiveData is used when you want to get updates in future that will happen on change of any object in DB. Also you may want to use LiveData when you want to execute the SELECT query asynchronously. It means you call getAnyMeal method and it does not block UI thread.
When you are using suspend keyword you can remove LiveData from return type. When suspend function is executed it will synchronously return you the result.
Solution
Update getAnyMeal() to the next form or create the new method as it is declared below:
#Query("SELECT * from meal_table LIMIT 1")
suspend fun getAnyMeal(): Array<Meal>
If you declare getAnyMeal method like this you will be able to call size property on the return type as it directly returns you an array.
On LiveData with Room:
Room doesn't support database access on the main thread unless you've called allowMainThreadQueries() on the builder because it might lock the UI for a long period of time. Asynchronous queries—queries that return instances of LiveData or Flowable—are exempt from this rule because they asynchronously run the query on a background thread when needed.
Observable queries with LiveData for more info.

Retrieving a HashMap LiveData via Room Library

I have a social networking app which displays a list of users, and am looking to have an efficient way of being able to retrieve an object from my LiveData using its primary key.
Example: Retrieve a set of User POJOs from within my LiveData<List<User>> given a LIST of userId Integers (ie, users 12, 5, 7, and 1). I need to be able to look up these users by the userId for display in the appropriate order in the UI.
I believe I want something more like LiveData<Map<Integer, User>>, but how could I implement this using the Room database, without breaking the LiveData callbacks from my local DB -> Room -> LiveData -> UI?
PROPOSAL 1:
Change my Room implementation to somehow return a LiveData containing a HashMap of <userId,User>.
Current Room implementation:
#Query("SELECT * FROM users WHERE user_id in :userIds LIMIT 1")
LiveData<List<User>> getUsers(List<Integer> userIds);
Proposed Room implementation (no idea if something like this is possible or what it would even look like):
#Query("SELECT * FROM users WHERE user_id in :userIds LIMIT 1")
LiveData<**HashMap**<Integer,User>> getUsers(List<Integer> userIds);
PROPOSAL 2:
Have a list of many LiveData objects WITHIN a Map:
Map<Integer,LiveData<User>> liveDataUsers;
This might be something to look into, but I'm worried that having potentially hundreds/thousands of LiveData objects within a map is bad design and could also lead to performance issues / too many open LiveData internal callback threads.
PROPOSAL 3:
Something else??? I feel like I am missing something easy here. How are others looking up objects within their LiveData using only their primaryKey?
edit: this is something I'd like to achieve at the Repo / Model level and not at the activity level, as this LiveData will be re-used throughout the app.

Create single onComplete for dynamic list of Completables

I'm using this library for wrapping Firebase transactions with RxJava. I'm new to RxJava, so this is mainly a question regarding how to use it.
Scenario: There is a many-to-many relationship between Persons and Labels. A Person can have multiple Labels, and a Label can be given to many Persons. When a Person is created, I must:
add them to the list of Persons
update each Label given to them to allow for querying all Persons that belong to a specific label
I have a list of Labels I want to write to my Firebase database.
List<Label> labels; // Let's assume it's been instantiated and added to
I want to write each of these to the DB:
FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference peopleRef = database.getReference().child("people");
DatabaseReference labelsRef = database.getReference().child("labels");
int newPersonId = peopleRef.push().getKey();
I can do this easily if I don't care about whether the calls are successful.
// Let's assume I already saved the Person to the DB
for (Label label : labels){
// For each label, index the Person saved (Looks like 'personId: true')
labelsRef.child(label).child(newPersonId).setValue(true);
}
But what if I do care about the result? If I want to react to all Labels being updated (like navigate away from the current Activity), I need to know if they've all been updated successfully.
RxFirebase is implemented such that setting a value in the DB returns a Completable. I essentially want to zip together n number of Completables and do something only when they succeed or fail.
So far, I can do this if I only want to update one Label, but I want to update n Labels.
The following code snippet chains 2 Completables together, but only saves 1 Label
RxFirebaseDatabase.setValue(peopleRef.child(newPersonId), person) // Save the Person
.andThen(RxFirebaseDatabase.setValue(labelsRef.child(label).child(newPersonId), true)) // I can index 1 Label, and this returns a Completable
How would I do this? If you know Firebase well enough, is this even the right way to be saving a List of items?

If I understood your main question correctly, you have a collection of Completable and you need to subscribe to them as one.
The way to solve this is using the Completable.concat or Completable.merge operators.
Completable.concat: Returns a Completable which completes only when all sources complete, one after another.
Completable.merge: Returns a Completable instance that subscribes to all sources at once and completes only when all source Completables complete or one of them emits an error.
Example:
List<Completable> tasks; // initialized elsewhere
Completable
.concat(tasks)
.subscribe(
() -> Log.d(TAG, "All successful"),
throwable -> Log.w(TAG, "One or more failed"))
About your second question, I don't know Firebase well enough.
Update: to obtain the List<Completable> you can do something similar to this:
List<Completable> tasks = new ArrayList<>();
for ( ... ) {
tasks.add(RxFirebaseDatabase.setValue(peopleRef.child(newPersonId), person));
}
Completable.concat(tasks).etc

Rxjava Filtering the duplicate item in List

I am using RxJava and Retrofit in My App.Here's the setup of the app.When the app is launched the app make two request one to the database and other to the Network Api (using Retrofit) and both request return a Observable<List<Article>>. So what I did is basically merged the two Observable. Now the problem is sometimes the network return Articles that are already present in the Database. So how do I filter out the duplicate item from the List. Here's my Code.
return Observable.merge(dataSource.getArticles(source), remoteSource.getArticles(source))
.distinct();
So I tried distinct operator but it's not filtering the Articles out.Here's the output looks like form db.
Article1
Article2
Article3
Article4
Output from Network
Article7
Articke8
Article3
Article4
What I want is a distinct list of Article

Assuming your Article has proper equals implementation,
you could collect them into a set:
dataSource.getArticles(source)
.mergeWith(remoteSource.getArticles(source))
.collect(HashSet::new, (set, list) -> set.addAll(list))
or you could unroll each list and apply distinct followed by toList:
dataSource.getArticles(source)
.mergeWith(remoteSource.getArticles(source))
.flatMapIterable(v -> v)
.distinct()
.toList()

That's because they are returning different lists. So the distinct method recognize them as different items
If you want to emit first the database items and then add the server ones... This may be a bit more complex but not too much ;)
Observable<List<Article>> databaseArticles = ...
Observable<List<Article>> serverArticles = ...
Observable<List<Article>> allArticles =
Observable.combineLatest(
databaseArticles,
serverArticles
.startWith(emptyList()), // so it doesn't have to wait until server response
(dbItems, sItems) => {
// Combine both lists without duplicates
// e.g.
Set<Article> all = new HashSet<>();
Collections.addAll(all, dbItems);
Collections.addAll(all, sItems);
return new ArrayList<>(all);
});

Chain flowable to completable with action

I have Android Room's SQL query, that return flowable:
#Query("SELECT * FROM exercices WHERE lang = 'ru' AND id_exercice = :id")
Flowable<Exercices> getExercicesById(int id);
In my repository, I need get emitted element, than change his boolean value, then call new method, that must returns completable.
That's what I try:
#Override
public Completable setExerciseUsed(int id) {
return mDatabase.exerciseDao().getExercicesById(id)
.doOnNext(exercise -> exercise.setIs_used(1))
.flatMapCompletable(exercise ->
Completable.fromAction(() -> mFitnessDatabase.exerciseDao().addExercise(exercise)));
}
Also i tried:
Exercices e = mDatabase.exerciseDao().getExercicesById(id).blockingFirst();
e.setIs_used(0);
return Completable.fromAction(() -> mDatabase.exerciseDao().addExercise(e));
But t not works properly. It seems like flowable emits many elements, and it going to stuck in cycle, after subscription.

Since your DAO returns Flowable, it will emit fresh data each time table is modified.
So after calling mFitnessDatabase.exerciseDao().addExercise(exercise),
getExercicesById will emit new data, thus the chain will execute forever.
If you want Room not to emit data - just change Flowable to Single.
Since you expect the one value to e returned, it's a good idea to limit the result to one item: "SELECT * FROM exercices WHERE lang = 'ru' AND id_exercice = :id LIMIT 1".
But actually, in your case, if you want to change a parameter of an item, it's much more efficient to do this within one query.
It may look like:
#Query("UPDATE exercices SET is_used = 1 WHERE lang = 'ru' AND id_exercice = :id")
fun setIsUsed(id: Int)

I don't know much about flowables or how to unsubscribe from one source with those, but have you thought about using LiveData? You can simply add a source to a LiveData object, then remove the connection to the source, change the object and execute your method without going into loop.
Another idea would it be that you retrieve your object, keep the connection and updating the value outside of that. Because you are keeping the database connection with LiveData, the observer would execute the onChanged again (because the source = database has been changed). You only would need to make sure that this method returns for example null (via setValue) for the time being until the database uploaded the new object value.
If you want to stay with flowables, maybe you should consider adding a condition which will prevent the app from re-applying the integer to the object (and sending it to the database). Does that makes sense to you?