#34944: Missing or misinferred attributes in output fields of generated fields
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Reporter: Paolo Melchiorre | Owner: Om Dahale
Type: Bug | Status: assigned
Component: Database layer | Version: 5.0
(models, ORM) |
Severity: Release blocker | Resolution:
Keywords: field, database, | Triage Stage: Accepted
generated, output_field |
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Paolo Melchiorre):
Replying to [comment:5 David Sanders]:
> Perhaps forum discussion is required ...
I started a discussion in the forum:
https://forum.djangoproject.com/t/release-blocker-missing-or-misinferred-
attributes-in-output-fields-of-generated-fields/25103
> ... I don't think we ought to go down the route of trying to make
`resolve_expression()` smart enough to determine that concat'ing 2
varchars means we need to add the max-lengths together ... 🤔
In fact, in my proposal for the "Cumulative proposal", I excluded this
hypothesis.
> We definitely need to fix the broken "None" in the DDL 👍
Yes, I think it absolutely needs to be resolved.
> > Always require specifying the output field (except when you are sure
that the extracted type cannot generate error situations?)
>
> Perhaps 👍 "Explicit is better than implicit" and all that, though I
think that documenting "For complex expressions consider always declaring
an output_field" is a nice option.
It could be a good solution, but it should be highlighted that deriving
the `output_field` from the expression could hide pitfalls even in
apparently simple cases such as concatenating two `CharField` on `SQLite`.
Not all Django developers look under the hood of the `ORM` or check the
`SQL` code generated by migrations.
Perhaps it might be safer to always require the user to specify an
`output_field`.
> I'm keen to hear Mariusz & Lily's thoughts.
Me too, and also the people who collaborated on this feature like Simon,
Lily, and Adam.
--
Ticket URL: <https://code.djangoproject.com/ticket/34944#comment:8>
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