not looking deeply but the hybrid you have in prop_3 doesn't seem to have
> any relationship to the base set of rows you're getting from "fractions".
> it returns multiple rows because statement2 isn't using any aggregates.
>
> How about a straight SQL string? what SQL do you expect? these are very
> easy to link to a hybrid.
>
>
It's difficult to issue a straight SQL string for the hybrid property
itself because what I'm working with are group_by constructs which depend
on aggregate functions. So, the hybrid property would need to be aware of
its context. For example, in the code below, prop_3 now returns the
maximum of all fractions and is not responsive to the group_by that comes
later down the line:
ECHO = False
from sqlalchemy.orm import sessionmaker, aliased
from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, select, func
from sqlalchemy.orm import relationship
from sqlalchemy.ext.hybrid import hybrid_property
engine = create_engine('sqlite:///:memory:', echo=ECHO)
Base = declarative_base()
class Fraction(Base):
__tablename__ = 'fractions'
id = Column(Integer, primary_key = True)
prop_1 = Column(Integer)
prop_2 = Column(Integer)
prop_3_num = Column(Integer)
prop_3_den = Column(Integer)
@hybrid_property
def prop_3(self) :
return 0
@prop_3.expression
def prop_3(self) :
alias_1 = aliased(Fraction)
alias_2 = aliased(Fraction)
statement1 = select([func.max(1.0 * alias_1.prop_3_num /
alias_1.prop_3_den).label('fmax')])
statement2 = select([alias_2.prop_3_num.label('prop_3_num_max'),
alias_2.prop_3_den.label('prop_3_den_max')]).\
where((1.0 * alias_2.prop_3_num /
alias_2.prop_3_den) == statement1)
return statement2
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()
for x in range(10) :
session.add(Fraction(prop_1=x%2, prop_2=x%4, prop_3_num = x+1,
prop_3_den=x+2))
session.commit()
for x in session.query(Fraction.prop_1, Fraction.prop_2, Fraction.prop_3) :
print x
print "########################"
for x in session.query(Fraction.prop_1, Fraction.prop_2,
Fraction.prop_3).group_by(Fraction.prop_1, Fraction.prop_2) :
print x
*******
the result is:
(0, 0, 10, 11)
(1, 1, 10, 11)
(0, 2, 10, 11)
(1, 3, 10, 11)
(0, 0, 10, 11)
(1, 1, 10, 11)
(0, 2, 10, 11)
(1, 3, 10, 11)
(0, 0, 10, 11)
(1, 1, 10, 11)
########################
(0, 0, 10, 11)
(0, 2, 10, 11)
(1, 1, 10, 11)
(1, 3, 10, 11)
whereas I'd like for the local maxima to be chosen, meaning:
(0, 0, 9, 10)
(0, 2, 7, 8)
(1, 1, 10, 11)
(1, 3, 8, 9)
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