code: public <T> Transformer<T, T> beginTransactionOnNext_() {
          return new Transformer<T, T>() {
              @Override
              public Observable<T> call(Observable<T> source) {
                  return beginTransactionOnNext(Database.this, source);
              }
          };
      }
docs: Starts a database transaction for each onNext call. Following database
  calls will be subscribed on current thread (Schedulers.trampoline()) and
  share the same {@link Connection} until transaction is rolled back or
  committed.
  
  @return begin transaction Transformer