Question

情况

用户进行购买，并将其作为交易存储在3个不同的表中（取决于类型）。我需要计算男女用户的交易/购买总额，所以我需要查看所有3个表。

为此，我在User表中创建了一个@property：

@property
def count_credits_purchases(self):
    trans = object_session(self).query(Transaction_1).filter(Transaction_1.type == "credits").with_parent(self).count()
    trans_vk = object_session(self).query(Transaction_2).filter(Transaction_2.type == "credits").with_parent(self).count()
    trans_stripe = object_session(self).query(Transaction_3).filter(Transaction_3.type == "credits").with_parent(self).count()
    value = trans + trans_vk + trans_stripe
    return int(value)

我正在尝试使用sqlalchemy func.sum()计算购买总额：

total_purchases_males_credits = db_session.query(func.sum(Users.count_credits_purchases))
.filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == "1")
.scalar()

问题

AttributeError: 'property' object has no attribute 'translate'

translate方法是字符串方法，这是怎么回事？我肯定会在count_credits_purchases中返回一个整数。

我进行了测试，并检查每个用户的值始终正确：

all_users = db_session.query(Users).limit(200)
for user in all_users:
    print (user.count_credits_purchases) # gives correct result

我可以创建一个变量并在循环中计算它，但是它效率极低，如果有5万用户，则可能需要1个小时。我需要了解如何使用@property属性

Answer 1

正如文档Using Descriptors and Hybrids所述，您应该使用hybrid_property才能在查询中使用它们。

看看文档中的示例：

class EmailAddress(Base):
    __tablename__ = 'email_address'

    id = Column(Integer, primary_key=True)

    # name the attribute with an underscore,
    # different from the column name
    _email = Column("email", String)

    # then create an ".email" attribute
    # to get/set "._email"
    @property
    def email(self):
        return self._email
虽然我们的EmailAddress对象将值通过电子邮件描述符传递到_email映射的属性中，但类级别EmailAddress.email属性没有Query可使用的常规表达语义。为了提供这些，我们改用hybrid扩展名

Answer 2

这里最好的解决方案可能是使用@hybrid_property，但我无法使其正常工作。

我使用经典方法提出了一个完全不同的解决方案。这非常快，到目前为止，我没有看到任何缺点：

# Normal method to calculate | Best case would probably be @hybrid_method
def count_credits_purchases(self, start_date, end_date, gender):
    trans = db_session.query(Transaction_1).filter(Transaction_1.type == "credits", Transaction_1.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
    trans_vk = db_session.query(Transaction_2).filter(Transaction_2.type == "credits", Transaction_2.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
    trans_stripe = db_session.query(Transaction_3).filter(Transaction_3.type == "credits", Transaction_3.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
    value = trans + trans_vk + trans_stripe
    return value

调用python：

total_purchases_males_credits = Users().count_credits_purchases(start_date, end_date, "1")

我仍然想知道与hybrid_property相比，这种方法有多好？

编辑：

也可以使用@hybrid_method：

@hybrid_method
def count_credits_purchases(self, start_date, end_date, gender):
    trans = db_session.query(Transaction_1).filter(Transaction_1.type == "credits", Transaction_1.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
    trans_vk = db_session.query(Transaction_2).filter(Transaction_2.type == "credits", Transaction_2.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
    trans_stripe = db_session.query(Transaction_3).filter(Transaction_3.type == "credits", Transaction_3.user_id == Users.id).filter(Users.date_added >= start_date, Users.date_added <= end_date, Users.gender == gender).count()
    value = trans + trans_vk + trans_stripe
    return value

并使用它：

total_purchases_males_credits = db_session.query(func.sum(Users.count_credits_purchases(start_date, end_date, "1"))).scalar()

SQLAlchemy AttributeError：'属性'对象没有属性'翻译'

2 个答案: