Title: Troubleshooting a SQLAlchemy conflict in the definition of the 'purchases' attribute in User and Purchase classes
Tags: python,flask,sqlalchemy,flask-sqlalchemy,flask-wtforms

Question: I am encountering an error while defining the relationship between the User and Purchase classes in SQLAlchemy. Both classes have an attribute named purchases that is used as a backref for the relationship. However, I have already made changes to resolve the conflict, but the error persists.
class Role(db.Model):
    __tablename__ = 'roles'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), unique=True)
    is_default = db.Column(db.Boolean, default=False, index=True)
    permissions = db.Column(db.Integer)
    users = db.relationship('User', backref='role', lazy='dynamic')

    @staticmethod
    def insert_roles():
        roles = {
            'User': (Permission.FOLLOW |
                     Permission.COMMENT |
                     Permission.WRITE_ARTICLES, True),
            'Moderator': (Permission.FOLLOW |
                          Permission.COMMENT |
                          Permission.WRITE_ARTICLES |
                          Permission.MODERATE_COMMENTS, False),
            'Administrator': (0xff, False)
        }
        for r in roles:
            role = Role.query.filter_by(name=r).first()
            if role is None:
                role = Role(name=r)
            role.permissions = roles[r][0]
            role.is_default = roles[r][1]
            db.session.add(role)
        db.session.commit()

    def __repr__(self):
        return '<Role %r>' % self.name


class User(UserMixin, db.Model):
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String(64), unique=True, index=True)
    username = db.Column(db.String(64), unique=True, index=True)
    role_id = db.Column(db.Integer, db.ForeignKey('roles.id'))
    password_hash = db.Column(db.String(128))
    confirmed = db.Column(db.Boolean, default=False)
    name = db.Column(db.String(64))
    purchases = db.relationship('Purchase', backref='user_purchases')
    
    def __repr__(self):
        return '<User %r>' % self.username


class Shoe(db.Model):
    __tablename__ = 'shoes'
    id = db.Column(db.Integer, primary_key=True)
    brand = db.Column(db.String(100))
    name = db.Column(db.String(100), nullable=False, unique=True)
    description = db.Column(db.String(1024))
    material = db.Column(db.String(100))
    price = db.Column(db.Integer, nullable=False)
    color = db.Column(db.String(50))
    size = db.Column(db.String(10), nullable=False)
    image = db.Column(db.String(255), nullable=False, unique=True)
    category_id = db.Column(db.Integer, db.ForeignKey('categories.id'))

    category = db.relationship('Category', backref='shoes')
    purchases = db.relationship('Purchase', back_populates='shoe')

    def __repr__(self):
        return '<Shoe %r>' % self.name


class Category(db.Model):
    __tablename__ = 'categories'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100), nullable=False)
    description = db.Column(db.String(1024))
    gender = db.Column(db.String(10), nullable=False, default='U')

    # Definición de las opciones de género como una lista de tuplas
    GENDER_CHOICES = [
        ('M', 'Masculino'),
        ('F', 'Femenino'),
        ('U', 'Unisex'),
        ('N', 'Niño'),
        ('G', 'Niña'),
    ]

    def __repr__(self):
        return f'Category {self.name}'


class Purchase(db.Model):
    __tablename__ = 'purchases'
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('users.id'))
    shoe_id = db.Column(db.Integer, db.ForeignKey('shoes.id'))

    user_purchases = db.relationship('User', backref='purchases')
    shoe = db.relationship('Shoe', backref='purchases')

    def __repr__(self):
        return '<Purchase %r>' % self.id

sqlalchemy.exc.ArgumentError: Error creating backref 'user' on relationship 'User.purchases': property of that name exists on mapper 'Mapper|Purchase|purchases'

Answer: I think you should remove user_purchases = db.relationship('User', backref='purchases') in class Purchase, because it already exists in the User class