> Can Django have a RealField that automatically does the cast?
I dont think it is a good idea to go that route in django and add that
as a standard django field, mainly for these reasons:
- FloatField happens to align correctly between pythons only "native"
float type and the chosen db counterparts (as both sides use double
precision under the hood), for a single precision field type that
congruence is not given anymore leading to even more errors
- there are database systems that only do double precision (sqlite I think)
- single precision is an edge case and quite hard to get maths running
with in python (you'd need to shim everything either with ctypes or
numpy, or write your own C-binding)
The last point raises the question - why should django provide a field
type, thats not even represented in its language? To me the django ORM
never felt like a database feature exporter at any cost, but like a
convenient way to interact with databases within typical python
paradigms. Here python clearly falls short lacking a single precision
type, so there are no common use cases at all for this. And mixing
precisions is never a good idea, as your example already showed.
Last but not least - you can still create a single precision float field
yourself, but you should also write a corresponding python type, so you
can do proper maths with it. In your field class you can use
get_db_prep_value or get_prep_value to customize the needed SQL repr
(eg. append inline cast), or use a custom psycopg OID adapter for your
python type.
Cheers,
jerch
Am 25.08.22 um 20:39 schrieb 'Zeev Tarantov' via Django developers
(Contributions to Django itself):
Hi!
I would like to provide an example of why this field is needed.
In PostgreSQL, a floating point literal like 4.6 has type decimal, not
binary floating point (see SELECT pg_typeof(4.6)).
When comparing binary floating point values to decimals, the decimal is
converted to double precision binary floating point value. This make
FloatField work.
But if you have real/float4 fields and you compare them to decimal
value, it doesn't work.
Here is an example, with a workaround.
in bar/models.py
from django.db import models
from django.db.models import FloatField
class RealField(FloatField):
def db_type(self, connection):
return 'real'
class Thing(models.Model):
score = RealField(null=False)
and now in shell:
from django.db.models import Value
from django.db.models.functions import Cast
from bar.models import RealField, Thing
Thing(score=4.6).save()
Thing.objects.all()[0].score
4.6
Thing.objects.filter(score=4.6).first()
None
str(Thing.objects.filter(score=4.6).query)
SELECT "bar_thing"."id", "bar_thing"."score" FROM "bar_thing" WHERE
"bar_thing"."score" = 4.6
Thing.objects.filter(score=Cast(Value(4.6), RealField())).first()
<Thing: Thing object (1)>
str(Thing.objects.filter(score=Cast(Value(4.6), RealField())).query)
SELECT "bar_thing"."id", "bar_thing"."score" FROM "bar_thing" WHERE
"bar_thing"."score" = (4.6)::real
Can Django have a RealField that automatically does the cast?
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