DecimalField truncates trailing zeros in the fractional part on SQLite

[Raphael Michel]

With Django 1.11, when reading the value of a DecimalField from the database, the Decimal is always returned with the precision of the DecimalField, i.e. if the field has 2 decimal places, I always get a Decimal with two decimal places as well.

With Django 2.0, I get a rounded decimal with no more decimal places.

Sample:

obj = Foo.objects.create(a="bar", d=Decimal('8.320'))
obj.refresh_from_db()

print(repr(obj.d)) 

This will output Decimal('8.320') on Django 1.11 but Decimal('8.32') on Django 2.0.

For me (and likely many others) this is quite critical: For example, if you use DecimalFields to store amounts of money in a currency that always has two places, you can just pass the databases to localize() and get a user-friendly representation. This is no longer possible, as you would first need to call quantize in every single place which would be quite an effort to do in a large codebasis.

Since there is no mention of this in the release notes, I believe this is an unwanted regression and hope it can be fixed in 2.0.2 or the like.

I wrote a ​regression test for the problem and ran git bisect to identify that the problem was introduced in commit ​a146b6562.

I did not find a way to keep the performance gain of this commit without introducing this regression, so I strongly ask to revert that commit altogether for now (and add my regression test, if wanted) until someone has a better idea how to optimize this.

[Simon Charette]

Thank you for your detailed report Raphael.

Could you confirm the issue is only present on SQLite? How do other database backend behave in this regard?

[Sergey Fedoseev]

​PR

[Tim Graham <timograham@…>]

In 6fd6d838:

Fixed #28915 -- Prevented SQLite from truncating trailing zeros in the fractional part of DecimalField.

This reverts commit a146b65628e702a9a3ed5be21542ca45366fbb29 and adds
a test for the regression.

[Tim Graham <timograham@…>]

In 0f7ca1e8:

[2.0.x] Fixed #28915 -- Prevented SQLite from truncating trailing zeros in the fractional part of DecimalField.

This reverts commit a146b65628e702a9a3ed5be21542ca45366fbb29 and adds
a test for the regression.

Backport of 6fd6d8383f48ea2fe4e058725fa30529a083e9a5 from master

[Tim Graham <timograham@…>]

In 7c939ae:

[2.0.x] Refs #28932 -- Skipped the failing test for refs #28915 on Oracle.