Python Flask framework. AssertionError: A setup function was called after the first request was handled

from flaskext.mysql import MySQL


def init_db_connection(app):
    mysql = MySQL()
    mysql.init_app(app)
    return mysql.get_db().cursor()

db=SQLAlchemy(app)

class Query:

  def __init__(self):
    SQLALCHEMY_DATABASE_URI = 'sqlite:///db/mydb.db'
    engine = create_engine(SQLALCHEMY_DATABASE_URI, connect_args={'check_same_thread': False})
    SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
    self.session = SessionLocal()
    Base = automap_base()
    Base.prepare(engine, reflect=True)
    self.Node = Base.classes.mytable



  def getTop20(self):
    nodes = self.session.query(self.Node).order_by(desc(self.Node.date_modified)).limit(20)
    return nodes