Hi All,
Say we have the following model:
class Price(Base):
__tablename__ = 'price'
id = Column(Integer, primary_key=True)
value = Column(Numeric(precision=36, scale=12))
class Instrument(Base):
__tablename__ = 'instrument'
id = Column(Integer, primary_key=True)
ticker = Column(String(50))
class Allocation(Base):
__tablename__ = 'data'
id = Column(Integer, primary_key=True)
trade_id = Column(Integer, index=True)
instrument_id = Column(ForeignKey(Instrument.id))
instrument = relationship(Instrument)
quantity = Column(Integer)
price_id = Column(ForeignKey(Price.id))
price = relationship(Price)
I now want to map the following class to the 'data' table:
class Trade(object):
__slots__ = (
'id', # Allocation.trade_id
'intrument_id', # Allocation.instrument_id
'quantity', # func.sum(Allocation.quantity)
'average_price',# see next line...
# (func.sum(Allocation.price)/func.sum(Allocation.quantity)
# Trades are mapped from Allocations by group_by(Allocation.trade_id)
)
...if that sort of makes sense?
I'm not quote sure how to map this, although I guess something along the
lines of
http://www.sqlalchemy.org/docs/mappers.html#mapping-a-class-against-arbitrary-selects
would be in order?
However, what that example doesn't show, and what I'm keen to make work,
is if someone changes attributes of the mapped Trade, what I'd like to
have happen depends on the attribute:
id,instrument_id, - all underlying rows are updated
quantity,average_price - an error is raised
Are there any examples of this?
Any recommendations? ;-)
cheers,
Chris
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