SQL custom SQLAlchemy query column

Question

I have a declarative table defined as follows:

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 request 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 does not replace the column amount.

Tried to do this for hours, and I don't want to use raw SQL.

Answer 1

Any query you make will not replace the original column amount. But you can load another column using the 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))

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

---

, , :
case when ... WHERE, :

1) , case when ... :

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

, setter:

@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) or_ (, ):

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

Answer 2

, , column_property. , . , , "" ?

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(...)