Many-to-one relation returns None object: SqlAlchemy

Question

I'm trying to write Schedule class which contains records like this: ... session, base, engine declaration somewhere here...

class Schedule(Base):
    __tablename__ = 'schedule'
    id = Column(Integer, primary_key=True)
    # and here i need ref to Station class
    station_id = Column(Integer, ForeignKey('station.id'))
    station = relationship('Station') # Also tried Station
    arr_time = Column(Time)

    def __init__(self, station_name, arrive_time):
         self.metadata.create_all()
         self.arrive_time = arrive_time

         # And now I'm trying to find station object by a given name
         # and add ref to it in self.station. 
         # The selection of a station works properly here:
         station = session.query(Station).filter(Station.name == station_name).first()
         # But on this statement I get an error: None object has no method 'append'
         self.station.append(station)
         session.add(self)
         session.commit()

After that I implement class 'Station'

class Station(Base):
    __tablename__ = 'stations'
    id = Column(Integer, primary_key=True)
    name = Column(String)

    def __init__(self, name):
         self.name = name

So, when I'm trying to add new Schedule record I get an error:

AttributeError: 'NoneType' object has no attribute 'append' 

The one-to-many case (where the foreign key in the first class and relationsip in the second one) works correctly.

What's wrong with my code?

Update: Also tried example from documentation:

engine = create_engine('sqlite:///:memory:')
Base = declarative_base(engine)
session = sessionmaker(bind=engine)()

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.id'))
    child = relationship("Child")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)


if __name__ == '__main__':
    Base.metadata.create_all()
    parent = Parent()
    session.add(parent)
    child = Child()
    session.add(child)
    print(hasattr(Parent, 'child'))
    print(hasattr(parent, 'child'))
    print(type(Parent.child))
    print(type(parent.child))

I get:

 >>> True
 >>> True
 >>> <class 'sqlalchemy.orm.attributes.InstrumentedAttribute'>
 >>> <class 'NoneType'>

Answer 1

I've got it)) The problem was that as default uselist flag in constructor of relationship is set for True. But - I don't really understand why it is not written in documentation - in the case of many-to-one relation this flag is set for False.

So, to solve my problem I simply have to change this:

station = relationship("Station", uselist=True)

or use in constructor:

self.station = station

This completely solves my problem.

Answer 2

I had a similar issue. I think the problem is that the relationship does not get triggered until you add and commit.

engine = create_engine('sqlite:///:memory:')
Base = declarative_base(engine)
session = sessionmaker(bind=engine)()

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.id'))
    child = relationship("Child")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)


if __name__ == '__main__':
    Base.metadata.create_all()
    parent = Parent()
    session.add(parent)
    child = Child()
    session.add(child)
    session.commit() # IMPORTANT: this is necessary for relationship to work
    print(hasattr(Parent, 'child'))
    print(hasattr(parent, 'child'))
    print(type(Parent.child))
    print(type(parent.child))

The only line I added is

        session.commit() # IMPORTANT: this is necessary for relationship to work