Handle constraints error messages defined in model

[sevdog]

In django 4.1 it was added to constraints a violation_error_message attribute which is used by django validation to provide an appropriate error message when the constraint is not respected.

Currently this field is not used by DRF and was not covered in #7438, which was created before this feature was introduced and therefore did not addressed it later.

It would be very useful to support using the violation_error_message as default message for UniqueTogetherValidator automatically created from model's constraints.

However there is an issue in this: because django uses % formatting while the DRF uses .format and this makes the two messages formats incompatible. Thus a solution should be found without introducing some kind of regression for "unique togheter" validation.

This feature is important because currently there is no easy way to customize the error message of a unique validator derived from a unique constraint.

[tomchristie]

because django uses % formatting while the DRF uses .format

Would you be able to add interlinking to the above so that we can compare & contrast?

(There was an attempt made to resolve this in #8863, which was reverted in #9330.)

[sevdog]

This is the "format" section in DRF UniqueTogetherValidator:

django-rest-framework/rest_framework/validators.py

Line 98 in 085b7e1

message = _('The fields {field_names} must make a unique set.')

django-rest-framework/rest_framework/validators.py

Lines 175 to 178 in 085b7e1

	if checked_values and None not in checked_values and qs_exists(queryset):
	field_names = ', '.join(self.fields)
	message = self.message.format(field_names=field_names)
	raise ValidationError(message, code='unique')

This is the "format" section of django BaseConstraint (from which is derived the UniqueConstraint):

https://github.com/django/django/blob/d658a3162fbeb68d148d1b2fcf4da4fe1437eddb/django/db/models/constraints.py#L22

https://github.com/django/django/blob/d658a3162fbeb68d148d1b2fcf4da4fe1437eddb/django/db/models/constraints.py#L74-L75

There is also a big difference in which elements are used for formatting:

• DRF uses field_names to tell which fields are to be unique together
• Django uses name which is the DB constraint name (since it is also used by CheckConstraint)