Use of relationship and ForeignKey modules in SQLAlchemy

Question

I'm a little confused over the use of the two modules from SQLAlchemy. This is the code I have:

Base = declarative_base()

class Restaurant(Base):
    __tablename__ = 'restaurant'
    id = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)

class MenuItem(Base):
    __tablename__ = 'menu_item'
    name =Column(String(80), nullable = False)
    id = Column(Integer, primary_key = True)
    description = Column(String(250))
    price = Column(String(8))
    course = Column(String(250))
    restaurant_id = Column(Integer,ForeignKey('restaurant.id'))
    restaurant = relationship(Restaurant)

I understand that ForeignKey is used to define the foreign key relationship between the restaurant_id column of menu_item table and the id column of restaurant table. But why then is restaurant = relationship(Restaurant) used?

Answer 1

restaurant_id will refer to an id (the column value). restaurant will refer to a Restaurant instance that will be lazy loaded from the db on access (or eager loaded if you set up the right stuff earlier). If you set backref on the relationship you can also access a list of MenuItem objects from a Restaurant.