#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:14 Simon Charette]:
> Resolving the `output_field` of combined expressions is a complex task
that the ORM only has limited support for #26355, #31506, #33397, this is
not a problem specific to `GenerateField`.
Thanks for pointing this out, it helped me put this issue in the right
perspective.
> It is particularly tricky in the case of `varchar(x) + varchar(y)` as
some backends restrict the allowed length of `varchar` (e.g. MySQL at 255)
...
I would add that, to complicate things, the restrictions on the various
types of fields also vary between different versions of the same database.
> > The SQL code for the generated column has an automatically inferred
max length of only 255 characters (varchar(255))
>
> ... In this particular case it's tempting to adjust the logic of
`Concat._resolve_output_field` to be smarter and generate a
`CharField(max_length=511)` but I suspect it might have unintended effect
and not something we'd like to include in 5.0 at this point.
> I suggest that we take the same approach here and expect the user to
specify an explicit `output_field`.
I totally agree, and thanks for rephrasing my initial proposal more
clearly.
> For the second part of the problem, `CharField(None)`, I think that the
addition of a check is in order. Adding a
`GeneratedField._check_output_field` that simply delegates to
`self.output_field.check(**kwargs)` seems like the way to go to catch all
misuse of `output_field`.
Thanks for pointing out this approach.
--
Ticket URL: <https://code.djangoproject.com/ticket/34944#comment:18>
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