database.py

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
        
engine = create_engine("postgresql://user:@host/schema")
Base = declarative_base()
Base.metadata.reflect(engine)

models.py

from sqlalchemy.orm import relationship, backref
from database import Base
        
class Bar(Base):
    __table__ = Base.metadata.tables['bars']
        
class Foo(Base):
    __table__ = Base.metadata.tables['foos']

    bar = relationship(Bar, primaryjoin='Foo.bar_id == Bar.bar_id')