UniqueConstraint validation error message conditional vs non-conditional

[David Sanders]

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

[Othniel Davidson]

nice one there

--
You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group.
To unsubscribe from this group and stop receiving emails from it, send an email to django-develop...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/CADyZw-5w94wvx26OVdSaDHGjKxRGvdnCZYeMOM-p4J9t9JUqMw%40mail.gmail.com.

[charettes]

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

[David Sanders]

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

To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/455d67d0-ca3e-4bc2-888f-10312dae8915n%40googlegroups.com.

[charettes]

Hello David

> Would it be possible to group the message by field in the same way as standard unique?

This should already work for constraints over a single field but not on the ones with multiple fields[0] which is covered by the suite[1] but it doesn't look like UniqueConstraint.validate is providing code="unique" which might be the source of the issue you are encountering?

[0] https://github.com/django/django/blob/07ebef566f751e172e266165071081c7614e2d33/django/db/models/base.py#L1443-L1447

[1] https://djangoci.com/job/django-coverage/HTML_20Coverage_20Report/

[David Sanders]

Hi Simon,

> This should already work for constraints over a single field but not on the ones with multiple fields[0] which is covered by the suite[1] but it doesn't look like UniqueConstraint.validate is providing code="unique" which might be the source of the issue you are encountering?

Yep that's correct. The code="unique" is only ever set from Model.unique_error_message() which, as you're already aware, is only called if there's no condition in UniqueConstraint.validate(). Updating one of the tests confirms this (see below).

Given that we'd like to avoid wording complications with conditions… looks like we could simply add code="unique" in UniqueConstraint.validate() as per:

index 49c7c91de9..61ca15c7c4 100644
--- a/django/db/models/constraints.py
+++ b/django/db/models/constraints.py
@@ -364,6 +364,8 @@ class UniqueConstraint(BaseConstraint):
                 if (self.condition & Exists(queryset.filter(self.condition))).check(
                     against, using=using
                 ):
-                    raise ValidationError(self.get_violation_error_message())
+                    raise ValidationError(
+                        message=self.get_violation_error_message(), code="unique"
+                    )
             except FieldError:
                 pass

Which then fixes the patched test:

index d4054dfd77..d433555f9f 100644
--- a/tests/constraints/tests.py
+++ b/tests/constraints/tests.py
@@ -569,8 +569,9 @@ class UniqueConstraintTests(TestCase):
             name=obj1.name, color="blue"
         ).validate_constraints()
         msg = "Constraint “name_without_color_uniq” is violated."
-        with self.assertRaisesMessage(ValidationError, msg):
+        with self.assertRaisesMessage(ValidationError, msg) as cm:
             UniqueConstraintConditionProduct(name=obj2.name).validate_constraints()
+        self.assertEqual(cm.exception.message_dict, {"name": [msg]})

     def test_validate(self):
         constraint = UniqueConstraintProduct._meta.constraints[0]

(pasting test failure)

======================================================================
FAIL: test_model_validation_with_condition (constraints.tests.UniqueConstraintTests)
Partial unique constraints are not ignored by
----------------------------------------------------------------------
Traceback (most recent call last):
    ...
AssertionError: {'__all__': ['Constraint “name_without_color_uniq” is violated.']} != {'name': ['Constraint “name_without_color_uniq” is violated.']}
- {'__all__': ['Constraint “name_without_color_uniq” is violated.']}
?   ^^ ^^^^

+ {'name': ['Constraint “name_without_color_uniq” is violated.']}
?   ^ ^^

If this is acceptable I could open a ticket / start a PR?

--
David

To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/d7130f45-9c78-4dfc-8a77-f49c98071fb2n%40googlegroups.com.

[Amol Rashinkar]

Then what I do..? 

To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/CADyZw-4%2B3F9Nx%2BPVhNoh8Su5bvBky4O%2BBdOzmZT1MYxVaWDVrA%40mail.gmail.com.

[charettes]

Hello David,

The proposed patch and test adjustments make sense to me, please open an associated ticket and PR.

Thanks for digging this through and working on a solution!

I think that an argument could be made for the `if e.code == "unique" and len(constraint.fields) == 1` branch in `Model.validate_constraints` to be entirely encapsulated in `UniqueConstraint.validate` (which knows about len(self.fields)) but focusing on making sure the proper code is attributed to the ValidationError instance is definitely an improvement.

Cheers,

Simon