SQLAlchemy自定义查询列 [英] SQLAlchemy custom query column

问题描述

我有一个这样定义的声明性表:

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))
    ...

查询应为:

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

我的代码是:

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

但是它不能代替amount列.

曾经尝试用几个小时来做​​到这一点，但我不想使用原始SQL.

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

推荐答案

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

Any query you do will not replace original amount column. But you can load another column using following query:

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))

这将不仅返回Transaction对象，而且返回tuple(Transaction, Decimal)

This will not return only Transaction objects, but rather tuple(Transaction, Decimal)

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

But if you want this property be part of your object, then:
Since your case when ... function is completely independent from the condition in WHERE, I would suggest that you change your code in following way:

1)向您的对象添加一个属性，该属性按照以下步骤进行case when ...检查:

1) add a property to you object, which does the case when ... check as following:

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

您还可以完全包装提供setter属性的金额的这种特殊处理:

You can completely wrap this special handling of the amount providing a setter property as well:

@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)修复您的查询，使其仅包含带有or_子句的filter子句(看起来您的查询根本不起作用):

2) fix your query to only have a filter clause with or_ clause (it looks like your query does not work at all):

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

这篇关于SQLAlchemy自定义查询列的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！