Hi Simon,
Cheers for the explanation.
I'm ok with the error message being the "constraint is violated" generic
message as I agree with what you're saying.
Would it be possible to group the message by field in the same way as
standard unique?
ie, would this be an idea?:
django.core.exceptions.ValidationError: {'test': ['Constraint “unique” is
violated.']}
This way the customised error message would be preserved.
This also leads to another question I had about whether it might be an idea
to allow overriding error messages for constraints in forms using the
error_messages dict… but I'll leave that for another post :)
Regards,
David
On Mon, 12 Sept 2022 at 11:53, charettes <charett...@gmail.com> wrote:
> Hello David,
>
> This is expected because Django doesn't have a way to express the
> constraint in words to present to the user when a condition, which could be
> complex, is provided.
>
> When no conditions are defined the metadata is easy to interpret to form a
> sentence out of ("Foo with this field0, field1, fieldN already exists")
> while when a condition is provided Q can define very complex criteria that
> only the developer is able to express in words. This is the same class of
> problem as trying to have a machine describe a text representation of a
> regular expression. It's doable for very simple cases but almost impossible
> for complex ones.
>
> We decided not to try to be clever about what the error message should be
> and opted to have a default that uses the constraint name when a condition
> is present while allowing developers to provide a `violation_error_message`
> at constraint initialization to have them express what a more appropriate
> error message should be.
>
> If we take your particular example, how should Django figure out what the
> error message should be for a UniqueConstraint of this form?
>
> UniqueConstraint(
> fields=["foo"],
> name="unique_uppercase_foo",
> condition=Q(test__upper=F("test"))
> )
>
> The developer who defined the constraint is in a much better position to
> provide an error that says that "Test with an upper case "foo" must be
> unique". If you're not convinced try with a complex condition involving OR
> and mixing transform comparison (e.g. a unique constraint only defined on
> palindromes)
>
> Cheers,
> Simon
>
> Le dimanche 11 septembre 2022 à 14:52:47 UTC-4, othnield...@gmail.com a
> écrit :
>
>> nice one there
>>
>> On Sun, Sep 11, 2022 at 3:36 PM David Sanders <shang.xia...@gmail.com>
>> wrote:
>>
>>> Hi folks (and in particular Simon Charette),
>>>
>>> I had a bit of a gotcha moment when a custom unique constraint
>>> validation message disappeared when I added a condition to it. I won't
>>> raise a ticket for this because it looks intentional from the constraint
>>> validation PR, but I wanted to seek clarification and whether either some
>>> adjustment to the behaviour could be made or clarification in the docs.
>>>
>>> Here's a boiled down example:
>>>
>>> Validating models with UniqueConstraint usually groups errors by field
>>> name:
>>>
>>> class Test(Model):
>>> test = CharField(max_length=255)
>>>
>>> class Meta:
>>> constraints = [
>>> UniqueConstraint(fields=["test"], name="unique"),
>>> ]
>>>
>>> Test.objects.create(test="abc")
>>> test = Test(test="abc")
>>> test.validate_constraints()
>>> django.core.exceptions.ValidationError: {'test': ['Test with this Test
>>> already exists.']}
>>>
>>> However if a condition is added to the UniqueConstraint the validation
>>> error is categorised as a non-field error:
>>>
>>> class Test(Model):
>>> test = CharField(max_length=255)
>>>
>>> class Meta:
>>> constraints = [
>>> UniqueConstraint(fields=["test"], name="unique",
>>> condition=~Q(test="")),
>>> ]
>>>
>>> Test.objects.create(test="abc")
>>> test = Test(test="abc")
>>> test.validate_constraints()
>>> django.core.exceptions.ValidationError: {'__all__': ['Constraint
>>> “unique” is violated.']}
>>>
>>> This affects the way custom error messages are defined (eg forms
>>> error_messages dict vs constraint violation_error_message)
>>>
>>> This behaviour is defined in UniqueConstraint.validate() and looks to
>>> be intentional from this advice from Simon:
>>> https://github.com/django/django/pull/14625#issuecomment-897275721
>>>
>>> I was wondering what the rationale for that was? Would it be possible to
>>> make the behaviour consistent? And if not - would a PR to clarify how
>>> ValidationErrors are raised for constraints in the docs be welcomed?
>>>
>>> Cheers,
>>> David
>>>
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