Validate UniqueConstraint on a non-field for a Django ModelFOrm

modelform_validate_unique.py

class MyModelForm(ModelForm):
  
    def validate_unique(self):
        """
        Call the instance's validate_unique() method and update the form's
        validation errors if any were raised.
        """
        exclude = self._get_validation_exclusions()
        # Don't exclude validation constraints on the basis of the Endpoint
        # field. The value is set before .is_valid() is called, so we want
        # to evaluate it.
        exclude.remove("endpoint")

        # Duplicate Model.validate_unique() so that we can
        # evaluate unique constraints that are set in Meta.constraints
        unique_checks, date_checks = self.instance._get_unique_checks(
            exclude=exclude,
            include_meta_constraints=True,
        )

        errors = self.instance._perform_unique_checks(unique_checks)
        date_errors = self.instance._perform_date_checks(date_checks)

        for k, v in date_errors.items():
            errors.setdefault(k, []).extend(v)

        if errors:
            # ModelForm.validate_unique catches the ValidationError from
            # Model.validate_unique then calls _update_errors()
            self._update_errors(errors)