Support violation_error_code and violation_error_message from UniqueConstraint in UniqueTogetherValidator

[s-aleshin]

Added support for retrieving custom violation_error_code (for django 5+) and violation_error_message from Django model-level UniqueConstraint definitions and passing them to UniqueTogetherValidator.

refs #9714 and #9352

The update ensures that:
• If violation_error_message or violation_error_code are explicitly defined on the model constraint, they are forwarded to the corresponding validator.
• If the constraint uses the default message/code, they are not passed, allowing the validator to use its own defaults.

Additionally, tests have been added to cover both scenarios:
• When a custom error message/code is used.
• When defaults are applied.

Note: The current structure of get_unique_together_constraints() may benefit from refactoring (e.g., returning a named structure for clarity and extensibility). This is outside the scope of this PR, but I may explore it in a future contribution.

[peterthomassen]

• If the constraint uses the default message/code, they are not passed, allowing the validator to use its own defaults.

What's the use case for that? (Why not use model-level default?)

[s-aleshin]

• If the constraint uses the default message/code, they are not passed, allowing the validator to use its own defaults.

What's the use case for that? (Why not use model-level default?)

Returning the constraint’s default message would change the output and might break users relying on the validator’s default message—for example, in tests or UI rendering. I consider that the current behaviour should be maintained.

[peterthomassen]

Looks good overall, just minor questions!

[peterthomassen]

lgtm! Let's wait for @browniebroke's comment on the open conversation and then we're done!

[s-aleshin]

lgtm! Let's wait for @browniebroke's comment on the open conversation and then we're done!

Hi @browniebroke! Just a quick ping in case this got lost — would be great to get your review when you have time.

[browniebroke]

Just a quick ping in case this got lost — would be great to get your review when you have time.

Thanks for the patience and pinging again, I indeed missed that. Will try to take a look this week

[browniebroke]

That looks good to me. Will have to wait a bit for merging at we are working out the next version to release (3.16.2 or 3.17) and when.

Thanks for your patience

[Copilot]

Pull Request Overview

This PR adds support for custom violation error codes and messages from Django model-level UniqueConstraint definitions to be passed through to UniqueTogetherValidator in Django REST Framework serializers.

• Enhanced UniqueTogetherValidator to accept and use custom error codes and messages
• Updated serializer logic to extract custom violation messages and codes from UniqueConstraint objects
• Added comprehensive test coverage for both custom and default error handling scenarios

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
rest_framework/validators.py	Enhanced UniqueTogetherValidator with code parameter support and updated error handling
rest_framework/serializers.py	Added constraint lookup logic and violation message extraction to pass custom codes/messages to validators
tests/test_validators.py	Added test model and test cases to verify custom and default error message/code behavior

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[peterthomassen]

lgtm!