single_ room.java

public Single<User> getUserById(String userId){
 return  db.getUserById(userId)
              /// if there is no user in the database get data from api
             .onErrorResumeNext(api.getUserById(userId)
              .subscribeOn(Schedulers.io())
              //check your request
              .filter(statusPojo::getStatus)
               // save data to room
              .switchMap(data -> {
              //sava data to db
              return Observable.just(data)
              })
           );

}