Hi,
I have the following many to one relation:
- A bank account table (acc)
- An Interest rate table, which relates to the account table. Colums are an
ID, a rate (decimal), a date and a flag outlining if the interest rate is
credit or debit ('H' / 'S')
- One account may have multiple interest rates (one to many)
What I now want is to retrieve the most recent (the current valid) interest
rate for a specific account. For now, I did this like this:
mapper_acc = mapper(Acc, table_acc, properties = {
# Current debit irate
'current_debit_irate': relation(
IRate, order_by = table_irate.c.date.desc(),
uselist = False, primaryjoin = and_(
table_acc.c.accid == table_irate.c.accid,
table_irate.c.type == "S"),
cascade="all")
})
This works but is very inefficient, as this mapper seems to read in all
interest rate objects despite I use "uselist=False", which is slow. So I
wonder if it's possible to optimize this in some way so that SQLAlchemy
constructs some specific SQL, something like:
"select * from irates where irateid = (select irateid, max(date) from irates
where accid = 123 and type = 'S' group by date)"
Any clues?
Best Regards,
Hermann
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