如何在SQLAlchemy中实现此混合属性的SQL级表达式？

Question

我有一个分类账表和一个相应的python类。我使用SQLAlchemy定义了模型，如下所示

class Ledger(Base):
    __tablename__ = 'ledger'

    currency_exchange_rate_lookup = {('CNY', 'CAD'): 0.2}

    amount = Column(Numeric(10, 2), nullable=False)
    currency = Column(String, nullable=False)
    payment_method = Column(String)
    notes = Column(UnicodeText)

    @hybrid_property
    def amountInCAD(self):
        if self.currency == 'CAD':
            return self.amount
        exchange_rate = self.currency_exchange_rate_lookup[(self.currency, 'CAD')]
        CAD_value = self.amount * Decimal(exchange_rate)
        CAD_value = round(CAD_value, 2)
        return CAD_value

    @amountInCAD.expression
    def amountInCAD(cls):
        amount = cls.__table__.c.amount
        currency_name = cls.__table__.c.currency
        exchange_rate = cls.currency_exchange_rate_lookup[(currency_name, 'CAD')]
        return case([
            (cls.currency == 'CAD', amount),
        ], else_ = round((amount * Decimal(exchange_rate)),2))

现在，正如您所看到的，我希望创建一个名为"amountInCAD“的混合属性。Python级别的getter似乎运行良好。但是，SQL表达式不起作用。

现在，如果我运行这样的查询：

>>>db_session.query(Ledger).filter(Ledger.amountInCAD > 1000)

SQLAlchemy给出了这个错误：

  File "ledger_db.py", line 43, in amountInCAD
    exchange_rate = cls.currency_exchange_rate_lookup[(currency_name, 'CAD')]
KeyError: (Column('currency', String(), table=<ledger>, nullable=False), 'CAD')

我研究了SQLAlchemy关于混合属性的在线文档。Expr.html#使用-a-混合将我的代码与示例代码进行比较，我不明白为什么我的代码不能工作。如果在官方示例中，cls.firstname可以引用值列，为什么在我的代码中cls.__table__.c.currency只返回一个Column而不是它的值？

Answer 1

cls.firstname不是“引用值”，而是Column。cls.firstname + " " + cls.lastname在示例中生成一个字符串连接SQL表达式，如下所示：

firstname || ' ' || lastname

这是混合属性的魔力之一:它们使编写可以在这两个域中工作的简单表达式相对容易，但在处理python实例和构建SQL表达式时仍然需要理解。

您可以重新考虑一下自己的混合操作，并将转换选项传递给case表达式中的DB：

from sqlalchemy import func

...

@amountInCAD.expression
def amountInCAD(cls):
    # This builds a list of (predicate, expression) tuples for case. The
    # predicates compare each row's `currency` column against the bound
    # `from_` currencies in SQL.
    exchange_rates = [(cls.currency == from_,
                       # Note that this does not call python's round, but
                       # creates an SQL function expression. It also does not
                       # perform a multiplication, but produces an SQL expression
                       # `amount * :rate`. Not quite sure
                       # why you had the Decimal conversion, so kept it.
                       func.round(cls.amount * Decimal(rate), 2))
                      for (from_, to_), rate in
                      cls.currency_exchange_rate_lookup.items()
                      # Include only conversions to 'CAD'
                      if to_ == 'CAD']
    return case(exchange_rates +  [
        # The default for 'CAD'
        (cls.currency == 'CAD', cls.amount),
    ])

这样，您就可以有效地将汇率查找作为CASE表达式传递给SQL。