id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
32555	DecimalField Rounding inconsistency (PostgreSQL)	bluppfisk	nobody	"Note, I've initially posted this on [https://stackoverflow.com/questions/66644473/django-decimalfield-inconsistent-rounding stackoverflow]

Django 2.2, PostgreSQL database. I have a `Line` object with a `positions` property. This is an `ArrayField` of a `DecimalField` with a `max_digits` of 12 and `decimal_places` of 3.

I store some floats as `Decimal`s and get them back out of the Database:

{{{
pos = [6586.87849502, 2.04190477e-01, 7.14666669e-01]
line = Line(positions=pos)
line.save()
line.refresh_from_db()  # send it through Django's ORM piping
print(line.positions)
}}}
Output:

> [Decimal('6586.87**9**'), Decimal('0.204'), Decimal('0.715')]

Interestingly, the first position was rounded up despite its next more significant digit being below 5. The other float is rounded down as expected.

I thought it might be PostgreSQL messing around, but no:

{{{
INSERT INTO line(positions) VALUES (array[6586.87849502, 2.04190477e-01, 7.14666669e-01]) RETURNING positions;
}}}

> positions (numeric[])
> {6586.878,0.204,0.715}


The real issue is that I should be able to predict what will come out of the database, down to the required precision of three decimal places:

{{{
[Decimal(x).quantize(Decimal(""0.001"")) for x in pos]
}}}

might yield

> [Decimal('6586.878'), Decimal('0.204'), Decimal('0.715')]

or

> [Decimal('6586.879'), Decimal('0.205'), Decimal('0.714')]

depending on the `decimal.ROUND_*` flag I pass in `quantize()`, but I never get consistent rounding throughout the array.

I also tried using the `django.db.backends.util::number_format` function for every of my `Decimal`s as I gathered that this is used by Django before inserting into the database, but the results are still inconsistent."	Bug	closed	Database layer (models, ORM)	2.2	Normal	duplicate	decimalfield, rounding, float	bluppfisk	Unreviewed	0	0	0	0	0	0