UniqueValidator error message not consistent with Django's

[Haj-Res]

The Django Rest Framework relies on Django's default error message for unique field validation, while allowing users the option to customize it. By default, the message is:

"%(model_name)s with this %(field_label)s already exists."

The issue arises when passing the parameters to format the string. Django passes the model_name and field_label parameters after applying the capfirst function to capitalize the first letter. However, the Rest Framework's get_unique_error_message function passes these parameters as they are, without modification. This wouldn’t be much of an issue if not for Django's convention, where verbose names for models and fields are lowercase by default and then capitalized programmatically when needed, allowing for more flexibility in messaging.

For example, when creating a user in my system, I receive an error message like: user with this email already exists.. However, if I used Django’s forms, the error message would be: User with this Email already exists..

There is no clean solution for capitalizing the error message when needed without changing every model's and field's verbose name to be capitalized, which goes against Django’s convention.

Here is the code from Django that prepares the parameters for formatting the error message before raising it:

    def unique_error_message(self, model_class, unique_check):
        opts = model_class._meta

        params = {
            "model": self,
            "model_class": model_class,
            "model_name": capfirst(opts.verbose_name),
            "unique_check": unique_check,
        }

        # A unique field
        if len(unique_check) == 1:
            field = opts.get_field(unique_check[0])
            params["field_label"] = capfirst(field.verbose_name)
            return ValidationError(
                message=field.error_messages["unique"],
                code="unique",
                params=params,
            )

And here is the Rest Framework doing the same:

def get_unique_error_message(model_field):
    unique_error_message = model_field.error_messages.get('unique', None)
    if unique_error_message:
        unique_error_message = unique_error_message % {
            'model_name': model_field.model._meta.verbose_name,
            'field_label': model_field.verbose_name
        }
    return unique_error_message

My suggestion is to modify the function used by the Rest Framework so that it calls capfirst on both the model_name and field_label strings. This would ensure the same behavior, with the model name capitalized by default.