Order of defining association object, related tables using Flask-SQLAlchemy?

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I'm working through Miguel Grinberg's Flask book.

In chapter 12, he has you define an association object Follow with followers and the followed, both mapping to a user, as well as adding followers and followed to the Users class.

I originally put the association table after the User table, and got an error when I ran python manage.py db upgrade:

line 75, in User followed = db.relationship('Follow', foreign_keys=  [Follow.follower_id], 
NameError: name 'Follow' is not defined

Then I moved the association object class Follow above the class User definition, and re-ran the migration. This time it worked.

Can someone explain the reason for this?

Both class definitions seem to need the other.

Is order something I should know about flask-sqlalchemy specifically, sqlalchemy, or ORM in general?

The SQLAlchemy documentation says "we can define the association_table at a later point, as long as it’s available to the callable after all module initialization is complete" and the relationship is defined in the class itself.

That is, for the case you're using and association_table to show the relationship between two separate models. I didn't see anything about this case in the Flask-SQLAlchemy or SQLAlchemy documentation, but it's very possible I just didn't recognize the answer when I saw it.

class User(UserMixin, db.Model):
    __tablename__ = 'users'
    ...
    followed = db.relationship('Follow',
                           foreign_keys=[Follow.follower_id],
                           backref=db.backref('follower', lazy='joined'),
                           lazy='dynamic',
                           cascade='all, delete-orphan')
    followers = db.relationship('Follow',
                            foreign_keys=[Follow.followed_id],
                            backref=db.backref('followed', lazy='joined'),
                            lazy='dynamic',
                            cascade='all, delete-orphan')

Order of definition with:

class Follow(db.Model):
    __tablename__ = 'follows'
    follower_id = db.Column(db.Integer, db.ForeignKey('users.id'), primary_key=True)
    followed_id = db.Column(db.Integer, db.ForeignKey('users.id'), primary_key=True)
    timestamp = db.Column(db.DateTime, default=datetime.utcnow)

Or maybe order doesn't matter at all, and I am misattributing a problem?

asked 41 secs ago

Han Lazarus

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Order of defining association object, related tables using Flask-SQLAlchemy?