Thanks:) I previously used cast instead of types_coerce for my use case.
On Friday, June 28, 2013 10:45:35 PM UTC+2, Michael Bayer wrote:
>
>
> On Jun 28, 2013, at 4:20 PM, Łukasz Fidosz <vir...@gmail.com <javascript:>>
> wrote:
>
> > Hi,
> > When subtracting DateTime columns in query, like:
> session.query((Foo.datetime1 - Foo.datetime2).label('diff')) SQLAlchemy
> tries to handle result as timedelta but MySQL returns float as a result so
> it crashes with "TypeError: unsupported operand type(s) for -: 'float' and
> 'datetime.datetime'" exception.
> > So I wondering if it's a bug or it should never be queried like that and
> proper function for subtracting datetimes should be always used instead?
> > Tested on fresh cloned from repository SQLAlchemy version.
> > Example code is attached to this message.
>
>
> its a missing feature right now. ideally we can add a rule to MySQL's
> datetime object that subtraction should return a type that will translate
> the float to an Interval (I'm guessing it's a number of days).
>
> You can get the float right now like this:
>
> from sqlalchemy import type_coerce
> session.query((type_coerce(Foo.date1 - Foo.date2,
> Float)).label('diff')).all()
>
>
> Or if on 0.8, a more comprehensive workaround like this:
>
> from sqlalchemy import type_coerce, Float, TypeDecorator
>
> class MySQLInterval(TypeDecorator):
> impl = Float
>
> def process_result_value(self, value, dialect):
> # guessing, seems to be close
> return datetime.timedelta(days=value / 100 / 60 / 60 / 24)
>
> class MySQLDateTime(TypeDecorator):
> impl = DateTime
>
> class comparator_factory(TypeDecorator.Comparator):
> def __sub__(self, other):
> return type_coerce(type_coerce(self.expr, DateTime) - other,
> MySQLInterval)
>
>
> Using MySQLDateTime will then treat the result of a __sub__() as a float
> to convert into a timedelta.
>
>
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