3

I have a declarative table defined like this:

class Transaction(Base):
    __tablename__ = "transactions"
    id = Column(Integer, primary_key=True)
    account_id = Column(Integer)
    transfer_account_id = Column(Integer)
    amount = Column(Numeric(12, 2))
    ...

The query should be: 

SELECT id, (CASE WHEN transfer_account_id=1 THEN -amount ELSE amount) AS amount
FROM transactions
WHERE account_id = 1 OR transfer_account_id = 1

My code is:

query = Transaction.query.filter_by(account_id=1, transfer_account_id=1)
query = query.add_column(case(...).label("amount"))

But it doesn't replace the amount column.

Been trying to do this with for hours and I don't want to use raw SQL.

4

2 回答 2

1

The construct you are looking for is called column_property. You could use a secondary mapper to actually replace the amount column. Are you sure you are not making things too difficult for yourself by not just storing the negative values in the database directly or giving the "corrected" column a different name?

from sqlalchemy.orm import mapper, column_property
wrongmapper = sqlalchemy.orm.mapper(Transaction, Transaction.__table,
    non_primary = True,
    properties = {'amount':
        column_property(case([(Transaction.transfer_account_id==1, -1*Transaction.amount)], 
        else_=Transaction.amount)})

Session.query(wrongmapper).filter(...)
于 2010-03-15T02:15:09.420 回答
1

您执行的任何查询都不会替换原始amount列。但是您可以使用以下查询加载另一列：

q = session.query(Transaction,
                  case([(Transaction.transfer_account_id==1, -1*Transaction.amount)], else_=Transaction.amount).label('special_amount')
                  )
q = q.filter(or_(Transaction.account_id==1, Transaction.transfer_account_id==1))

这不会只返回Transaction对象，而是tuple(Transaction, Decimal)

但是，如果您希望此属性成为您的对象的一部分，那么：
由于您的case when ...函数完全独立于 中的条件WHERE，我建议您按以下方式更改代码：

1）向您的对象添加一个属性，该属性进行case when ...如下检查：

@property
def special_amount(self):
    return -self.amount if self.transfer_account_id == 1 else self.amount

您也可以完全包装这种对提供 setter 属性的金额的特殊处理：

@special_amount.setter
def special_amount(self, value):
    if self.transfer_account_id is None:
        raise Exception('Cannot decide on special handling, because transfer_account_id is not set')
    self.amount = -value if self.transfer_account_id == 1 else value

2）将您的查询修复为只有一个带有子句的过滤子句or_（看起来您的查询根本不起作用）：

q = session.query(Transaction).filter(
    or_(Transaction.account_id==1, 
        Transaction.transfer_account_id==1)
)

# then get your results with the proper amount sign:
for t in q.all():
    print q.id, q.special_amount
于 2010-03-15T17:30:08.103 回答