text: Title: flask-sqlalchemy session close seems not work
  Body: I have a question about flask-sqlalchemy, precisely about sqlalchemy. 
  When executing one function, processes are recorded in database. 
  Whenever after recording db, I added db.session.close() to get session back to pool. 
  but while function is executed, I cannot connect to database. why is it happening ?
  
      def func(self):
          # stage 1:
          self.sub_func1() ->update process to db
  
          # stage 2:
          self.sub_func2() ->update process to db
  
          # stage 3:
          self.sub_func3() ->update process to db
  
          # stage 4:
          self.sub_func4() ->update process to db
  
          return result