AttributeError: '_Option' object has no attribute '_sa_instance_state' in flask

I am using Flask for my app and getting AttributeError: '_Option' object has no attribute '_sa_instance_state' error. have no idea what it is about. I google it and it seems that it is a SQL-Alchemy issue. Could you please help me with that? here is my models.py: ``` #current table is used to make navigation hierarchy. eg. menu/submenu #eg. About Us (History, Mission, Vision) menu_hierarchy = db.Table('menu_hierarchy', db.Column('parent_id', db.Integer, db.ForeignKey('menu.id')), db.Column('child_id', db.Integer, db.ForeignKey('menu.id')) ) class Menu(db.Model): """Menu is used for websites navigation titles. eg. Home/About Us/Blog/Contacts/and etc""" id = db.Column(db.Integer, primary_key = True) title = db.Column(db.String(255)) title_eng = db.Column(db.String(255)) alias = db.Column(db.String(255)) menu_type = db.Column(db.String(10)) ordering = db.Column(db.SmallInteger, default = '1') check_out_time = db.Column(db.DateTime) access = db.Column(db.String(30)) published = db.Column(db.SmallInteger, default = '1') parent_id = db.relationship('Menu', secondary = menu_hierarchy, primaryjoin = (menu_hierarchy.c.parent_id == id), secondaryjoin = (menu_hierarchy.c.child_id == id), backref = db.backref('menu_hierarchy', lazy = 'dynamic'), lazy = 'dynamic') content = db.Column(db.String) content_eng = db.Column(db.String) image = db.Column(db.String(350)) ```