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[英]Converting a SQLAlchemy @hybrid_property into a SQL .expression
[英]How to implement SQL level expression for this hybrid property in SQLAlchemy?
我有一个分类帐表和一个对应的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))
现在您可以看到,我想创建一个名为“ amountInCAD”的混合属性。 Python级别的获取器似乎工作正常。 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给我这个错误:
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')
我研究了有关混合属性的SQLAlchemy在线文档。 http://docs.sqlalchemy.org/en/latest/orm/mapped_sql_expr.html#using-a-hybrid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起作用。 如果在官方示例中, cls.firstname
可以引用值的列,为什么在我的代码中cls.__table__.c.currency
仅返回Column
而不是其值?
cls.firstname
不是“引用值”,而是“ Column
。 该示例中的cls.firstname + " " + cls.lastname
沿着以下行生成字符串连接SQL表达式:
firstname || ' ' || lastname
这是混合属性的魔力的一部分:它们使编写可以在两个域中都可以使用的简单表达式相对容易,但是在处理python实例和构建SQL表达式时,您仍然必须了解。
您可以重新考虑一下自己的混合动力,然后在case
表达式中将转换选项实际传递给数据库:
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,您就可以将汇率查询作为CASE
表达式有效地传递给SQL。
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