code: public Observable<Boolean> beginTransaction(Observable<?> dependency) {
          return update("begin").dependsOn(dependency).count().map(Functions.constant(true));
      }
docs: Starts a transaction. Until commit() or rollback() is called on the
  source this will set the query context for all created queries to be a
  single threaded executor with one (new) connection.
  
  @param dependency
  @return