How to handel no results with Android Room and RxJava 2?

Question

I have database with table contact and I want to check if there is contact with some phone number.

@Query("SELECT * FROM contact WHERE phone_number = :number") Flowable<Contact> findByPhoneNumber(int number); 

I have RxJava 2 Composite disposable with statement from above to check if there is contact with phone number.

disposable.add(Db.with(context).getContactsDao().findByPhoneNumber(phoneNumber)             .subscribeOn(Schedulers.io())             .observeOn(AndroidSchedulers.mainThread())             .subscribeWith(new DisposableSubscriber<Contact>() {                 @Override                 public void onNext(final Contact contact) {                     Log.d("TAG", "phone number fined");                     Conversation conversation;                     if(contact != null){                         conversation = Db.with(context).getConversationsDao().findBySender(contact.getContactId());                         if(conversation != null){                             conversation.setUpdatedAt(Utils.getDateAndTimeNow());                             saveConversation(contact, conversation, context, text, phoneNumber, false);                         } else {                             conversation = getConversation(contact, contact.getPhoneNumber());                             saveConversation(contact, conversation, context, text, phoneNumber, true);                         }                     } else {                         conversation = Db.with(context).getConversationsDao().findByPhone(phoneNumber);                         if(conversation != null){                             conversation.setUpdatedAt(Utils.getDateAndTimeNow());                             saveConversation(contact, conversation, context, text, phoneNumber, false);                         } else {                             conversation = getConversation(contact, phoneNumber);                             saveConversation(contact, conversation, context, text, phoneNumber, true);                         }                     }                 }                  @Override                 public void onError(Throwable t) {                     Log.d("TAG", "find phone number throwable");                     Toast.makeText(context, t.getLocalizedMessage(), Toast.LENGTH_LONG).show();                 }                  @Override                 public void onComplete() {                     Log.d("TAG", "onComplete");                 }             })); 

This is working fine if query can find contact with required phone number, but if there is result, it nothing happens.

Here are two test cases that I wrote and they work fine:

@RunWith(AndroidJUnit4.class) public class ContactsTest {      private AppDatabase db;      @Rule     public InstantTaskExecutorRule instantTaskExecutorRule =             new InstantTaskExecutorRule();      @Before     public void initDb() throws Exception {         db = Room.inMemoryDatabaseBuilder(                 InstrumentationRegistry.getContext(),                 AppDatabase.class)                 // allowing main thread queries, just for testing                 .allowMainThreadQueries()                 .build();     }      @After     public void close(){         db.close();     }      @Test     public void insertAndFindTest(){         final Contact contact = new Contact();         contact.setName("Test");         contact.setPhoneNumber(555);         db.contactsDao()                 .insert(contact);          db.contactsDao().findByPhoneNumber(contact.getPhoneNumber())                 .test()                 .assertValue(new Predicate<Contact>() {                     @Override                     public boolean test(@NonNull Contact savedContact) throws Exception {                         if(savedContact.getPhoneNumber() == contact.getPhoneNumber()){                             return true;                         }                         return false;                     }                 });     }      @Test     public void findNoValues(){         db.contactsDao().findByPhoneNumber(333)                 .test()                 .assertNoValues();     }  } 

How I can solve this?

Answer 1

As said here, you can use Maybe or Single for this case:

Maybe

@Query("SELECT * FROM Users WHERE id = :userId") Maybe<User> getUserById(String userId); 

Here’s what happens:

• When there is no user in the database and the query returns no rows, Maybe will complete.
• When there is a user in the database, Maybe will trigger onSuccess and it will complete.
• If the user is updated after Maybe was completed, nothing happens.

Single

@Query("SELECT * FROM Users WHERE id = :userId") Single<User> getUserById(String userId); 

Here are some scenarios:

• When there is no user in the database and the query returns no rows, Single will trigger onError(EmptyResultSetException.class)
• When there is a user in the database, Single will trigger onSuccess.
• If the user is updated after Single.onComplete was called, nothing happens, since the stream was completed.

It was added in version 1.0.0-alpha5.