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'>

Also get this error with example from documentation – Mikhail Elizarev Oct 14 '13 at 08:46 — Mikhail Elizarev, Oct 14 '13 at 08:46

score 4 · Accepted Answer · answered Oct 14 '13 at 09:54

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.

score 2 · Answer 2 · answered Jun 14 '16 at 22:08

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

Kobi K · Answer 3 · 2013-10-14T09:12:57.133

0

station = session.query(Station).filter(Station.name == station.name).first()

self.station is None....

Try to use debugger or just print station , print self.station

Edit: Class Variable Scopes:

There are 2 ways for class scope vars:

Instance variable - when using self, this is called instance variable and by that to each instance of that class you will have a copy of the variable.
Class attribute - in your case it's Schedule.station it's owned by the class, not the instances of the class.

Please refer to the Docs about namespace it will clear your problem.

edited Oct 14 '13 at 09:12

answered Oct 14 '13 at 08:44

Kobi K

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No, station in not None))) And even if it is None, you are able to add it to any list: `x = []; x.append(None)` gives `[None]`. Error exists on `relationsip`, not on station itself. – Mikhail Elizarev Oct 14 '13 at 08:48
this is the line that gives the error self.station.append(station) isn't it? that's what i understand from AttributeError: 'NoneType' object has no attribute 'append' – Kobi K Oct 14 '13 at 08:50
sorry, I have misspelling in my code example here. `Station.name == station_name`, not `station.name`. In my code everything properly (I've typed this code here, not copied) – Mikhail Elizarev Oct 14 '13 at 08:54
np:) just edit the post so it will be clear. also do you have self.station? and what is it's content? – Kobi K Oct 14 '13 at 08:56
self.station - is in fact request to the Schedule.station member, as I understand. Isn't it correct usage? – Mikhail Elizarev Oct 14 '13 at 09:01
Well, if i try to use self.classmember I will get a copy of class member in my instance. So it should be of type ``, such as in class. But in fact I get ``. I.e. property exists, but it is not defined. – Mikhail Elizarev Oct 14 '13 at 09:17
I've added example from sqlalchemy documentation with exactly the same problem – Mikhail Elizarev Oct 14 '13 at 09:28

Many-to-one relation returns None object: SqlAlchemy

3 Answers3