Question

我在界定<代码>User和-Purchase在Alchemy中的班级之间的关系时遇到了一个错误。两个班级都有一个名称:purchases。这被用作关系背书。然而,我已经为解决冲突做出了改变,但错误依然存在。

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

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