Chris Withers wrote:
> Michael Bayer wrote:
>> Chris Withers wrote:
>>> Michael Bayer wrote:
>>>>> Has anyone (hi, list, talking to you too!) already done a custom type
>>>>> for this specific problem?
>>>> people do custom types for all sorts of things. In the case of the
>>>> Decimal here I'd likely subclass sqlalchemy.databases.sqlite.SLNumeric
>>>> which should ensure that your own bind_processor() and
>>>> result_processor()
>>>> methods can be called.
>>> Okay, but how do I make sure this is only used when sqlite this the
>>> engine?
>>
>> You can use a TypeDecorator to switch between implementations. I had a
>> plan to add a pre-fab type for this to core that allows easy switching
>> of
>> implementations.
>
> Okay, this is what I've ended up with for this one:
>
> from decimal import Decimal
> from sqlalchemy import types
> from sqlalchemy.databases.sqlite import SQLiteDialect
>
> class Numeric(types.TypeDecorator):
> """A numeric type that respects precision with SQLite
>
> - always returns Decimals
> - always rounds as necessary as a result
> """
>
> impl = types.Numeric
>
> def bind_processor(self, dialect):
> if isinstance(dialect,SQLiteDialect):
> def process(value):
> if value is None:
> return None
> else:
> return float(value)
> return process
> else:
> return super(Numeric,self).bind_processor(dialect)
>
> def result_processor(self, dialect):
> if isinstance(dialect,SQLiteDialect):
> fstring = "%%.%df" % self.impl.scale
> def process(value):
> if value is None:
> return None
> else:
> return Decimal(fstring % value)
> return process
> else:
> return super(Numeric,self).result_processor(dialect)
>
> What should I have done differently and why?
>
> cheers,
>
> Chris
>
> PS: While looking into how to do this for 0.5.8, I saw this dire warning
> in 0.6:
>
> util.warn("Dialect %s+%s does *not* support Decimal objects natively, "
> "and SQLAlchemy must convert from floating point - "
> "rounding errors and other issues may occur. "
> "Please consider storing Decimal numbers as strings or "
> "integers on this platform for lossless storage." %
> (dialect.name, dialect.driver))
>
> ...which I also saw applies to SQLite.
>
> What are the "rounding errors and others issues" that you allude to?
> What is the integer/string recommendation saying? Use a String column?
> Use an Integer column? Is this something I could work into a
> TypeDecorator? Should I?
the whole ugly discussion is at http://www.sqlalchemy.org/trac/ticket/1759
>
> cheers,
>
> Chris
>
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