got error for python app

sqlalchemy.exc.InvalidRequestError: One or more mappers failed to initialize - can't proceed with initialization of other mappers. Triggering mapper: 'Mapper|Room|room'. Original exception was: relationship 'building' expects a class or a mapper argument (received: )

heres my code for the db

models.py

from app import db

class Building(db.Model): __tablename__ = "building"

id = db.Column(db.Integer, primary_key=True) name = db.Column(db.String)

def __repr__(self): return "".format(self.name)

class Room(db.Model): """""" __tablename__ = "room"

id = db.Column(db.Integer, primary_key=True) roomnumber = db.Column(db.String) ranges = db.Column(db.String) room_type = db.Column(db.String)

building_id = db.Column(db.Integer, db.ForeignKey("building.id")) building = db.relationship("building", backref=db.backref( "room", order_by=id), lazy=True)

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make_db.py

from sqlalchemy import create_engine, ForeignKey from sqlalchemy import Column, Integer, String from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import relationship, backref

engine = create_engine('sqlite:///ucsb_room.db', echo=True) Base = declarative_base()

class Building(Base): __tablename__ = "building"

id = Column(Integer, primary_key=True) name = Column(String)

def __repr__(self): return "{}".format(self.name)

class Room(Base): """""" __tablename__ = "room"

id = Column(Integer, primary_key=True) roomnumber = Column(String) ranges = Column(String) room_type = Column(String)

building_id = Column(Integer, ForeignKey("building.id")) building = relationship("building", backref=backref( "room", order_by=id))

Base.metadata.create_all(engine)