#36030: Expressions that divide an integer field by a constant decimal.Decimal
returns inconsistent decimal places on SQLite
-------------------------------------+-------------------------------------
Reporter: Bartłomiej Nowak | Owner: Gregory
| Mariani
Type: Bug | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: division | Triage Stage: Accepted
decimalfield |
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Jacob Walls):
I was using some shorthand in my reply, so it's totally fair to ask for
clarification (happy to), but it looks like you echoed back LLM output
mostly verbatim. I can do that myself; it doesn't need to be posted on
Trac. I would have appreciated you condensing your reply and writing it in
your own voice.
> What the ticket is highlighting is that the ORM currently doesn’t
preserve that invariant:
Actually, it's not the ORM, it's the database adapter.
Django sends `Decimal` objects to psycopg2. For the query
`Book.objects.filter(price=42).annotate(new_price=F("pages") /
Decimal(28))`, the ORM sends `Decimal` objects to the adapter:
{{{#!py
(Pdb) params
(Decimal('28'), Decimal('42'))
(Pdb) sql
'SELECT ("annotations_book"."pages" / %s) AS "new_price" FROM
"annotations_book" WHERE "annotations_book"."price" = %s LIMIT 21'
}}}
... and psycopg2 makes the reasonable choice to mogrify `Decimal(28)` to
"28". If that is undesired, there are multiple ways to fix this, including
adding an explicit cast, or by using string formatting to preserve the
precision.
I don't think `Decimal(28)` or `Decimal(float_var)` is standard usage that
we should memorialize in the docs. We don't document all database quirks.
This ticket should be enough. Hope that helps.
--
Ticket URL: <https://code.djangoproject.com/ticket/36030#comment:25>
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