Undefined attribute error for accessing relation

Question

I have defined two model with relation between them as below:

class Role(db.Model):

    __tablename__ = 'roles'

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(40), nullable=False, unique=False)
    db.relationship('User', backref='role', lazy='dynamic')

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

    def __repr__(self):
        return f'<Role id={self.id} name={self.name}>'


class User(db.Model):

    __tablename__ = 'users'

    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(40), unique=True)
    password = db.Column(db.String(40), unique=True)
    role_id = db.Column(db.Integer, db.ForeignKey('roles.id'))

    def __init__(self, username, password, role_id):
        self.username = username
        self.password = password
        self.role_id = role_id

    def __repr__(self):
        return f'<User id={self.id} username={self.username} password={self.password}>'

Then inside shell I created entries as below:

> admin_role = Role('Admin')
> db.session.add(admin_role)
> db.session.commit()
> admin_user = User('adminusername', 'adminpassword',admin_role.id)
> db.session.add(admin_user)
> db.session.commit()

When I try to query model I get perfect result:

>>> admin_role = Role.query.first()
>>> admin_user = User.query.first()
>>> print(admin_role)
<Role id=1 name=Admin>
>>> print(admin_user)
<User id=1 username=adminusername password=adminpassword>

But when I try to access relation

print(admin_role.users)
print(admin_user.role)

I get errors Role object has no attribute users and User object has no attribute role respectively.

Answer 1

Typo? You have to assign db.relationship() instance to a variable.

class Role(db.Model):

    __tablename__ = 'roles'

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(40), nullable=False, unique=False)
-    db.relationship('User', backref='role', lazy='dynamic')
+    users = db.relationship('User', backref='role', lazy='dynamic')