Thank you Simon. Your explanation helps me understand this quite a lot.
Sometimes the documentation is so terse that only when you fully understand
the subject then you can understand it by reading it:) But still if I want
to implement this hybrid property from the query level, how would you
suggest modify my current code? Or maybe you could please point out a link
to where I can explore further on the python to SQL transition? Thank you
so much.
Jinghui
On Mon, Oct 3, 2016 at 1:27 AM, Simon King <si...@simonking.org.uk> wrote:
> On Mon, Oct 3, 2016 at 7:17 AM, Jinghui Niu <niujing...@gmail.com> wrote:
> > I have a ledger table and a corresponding python class. I defined the
> model
> > using SQLAlchemy, as follows,
> >
> > class Ledger(Base):
> > __tablename__ = 'ledger'
> >
> > currency_exchange_rate_lookup = {('CNY', 'CAD'): 0.2}
> >
> > amount = Column(Numeric(10, 2), nullable=False)
> > currency = Column(String, nullable=False)
> > payment_method = Column(String)
> > notes = Column(UnicodeText)
> >
> > @hybrid_property
> > def amountInCAD(self):
> > if self.currency == 'CAD':
> > return self.amount
> > exchange_rate = self.currency_exchange_rate_
> lookup[(self.currency,
> > 'CAD')]
> > CAD_value = self.amount * Decimal(exchange_rate)
> > CAD_value = round(CAD_value, 2)
> > return CAD_value
> >
> > @amountInCAD.expression
> > def amountInCAD(cls):
> > amount = cls.__table__.c.amount
> > currency_name = cls.__table__.c.currency
> > exchange_rate = cls.currency_exchange_rate_
> lookup[(currency_name,
> > 'CAD')]
> > return case([
> > (cls.currency == 'CAD', amount),
> > ], else_ = round((amount * Decimal(exchange_rate)),2))
> >
> > Now as you can see, I want to create a hybrid property called
> "amountInCAD".
> > The Python level getter seems to be working fine. However the SQL
> expression
> > doesn't work.
> >
> > Now if I run a query like this:
> >
> >>>>db_session.query(Ledger).filter(Ledger.amountInCAD > 1000)
> >
> > SQLAlchemy gives me this error:
> >
> > File "ledger_db.py", line 43, in amountInCAD
> > exchange_rate = cls.currency_exchange_rate_lookup[(currency_name,
> > 'CAD')]
> > KeyError: (Column('currency', String(), table=<ledger>, nullable=False),
> > 'CAD')
> >
> > I've researched SQLAlchemy's online documentation regarding hybrid
> > property.http://docs.sqlalchemy.org/en/latest/orm/
> mapped_sql_expr.html#using-a-hybrid
> > Comparing my code to the example code, I don't understand why mine
> doesn't
> > work. If in the official example, cls.firstname can refer to a column of
> > value, why in my code the cls.__table__.c.currencyonly returns a Column
> not
> > its value?
>
> Forget about using this in a query for a second. For example, open a
> python shell, import your class, and type "Ledger.amountInCAD". This
> will trigger the same exception. There's no way that
> cls.__table__.c.currency can *ever* return a value from a specific
> row, because you are accessing it from the *class*, which isn't
> related to any specific row.
>
> I assume the example in the docs you are referring to is this one:
>
> @hybrid_property
> def fullname(self):
> return self.firstname + " " + self.lastname
>
> In this example, "User.fullname" is precisely equivalent to:
>
> User.firstname + " " + User.lastname
>
> User.firstname is not the value from any particular row - it's the
> (ORM-level) column object. The result of that expression is another
> SQL expression.
>
> Sorry, that's probably not a very good explanation. Has it made it any
> clearer?
>
> Simon
>
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