Add message_key to UniqueConstraintValidator for Structured Error Handling

[m16bappi]

Problem:

The UniqueTogetherValidator previously raised validation errors as plain messages. This caused issues when dealing with non_related_fields, leading to inconsistencies in error structures. As a result:

• Errors were difficult to associate with specific fields in frontend applications.
• Validation messages were not properly structured, making it challenging to integrate with form-handling logic.
• When non_related_fields were involved, the error response did not clearly indicate the affected field.

Solution:

• Introduced a message_key parameter to the UniqueTogetherValidator.
• If provided, the validation error will be returned as a dictionary with message_key as the key.
• If message_key is not provided, the error will default to {non_related_field: "The fields race_name, position must make a unique set."} for clarity.

Example

Before (Without message_key)

class ErrorMessageKeySerializer(serializers.ModelSerializer):
    class Meta:
        model = UniquenessTogetherModel
        fields = '__all__'
        validators = [
            UniqueTogetherValidator(
                queryset=UniquenessTogetherModel.objects.all(),
                fields=['race_name', 'position']
            )
        ]

Response:

{
  "non_related_field": ["The fields race_name, position must make a unique set."]
}

After (With message_key="name")

class ErrorMessageKeySerializer(serializers.ModelSerializer):
    class Meta:
        model = UniquenessTogetherModel
        fields = '__all__'
        validators = [
            UniqueTogetherValidator(
                queryset=UniquenessTogetherModel.objects.all(),
                fields=['race_name', 'position'],
                message_key='name'
            )
        ]

Response:

{
  "name": ["The fields race_name, position must make a unique set."]
}