public function determineValidationRules(ModelRelationData $relation, ModelFormFieldData $field)
    {
        $rules = [];

        if ($field->required() && ! $field->translated()) {
            $rules[] = 'required';
        } else {
            // Anything that is not required should by default be explicitly nullable
            // since Laravel 5.4.
            $rules[] = 'nullable';
        }

        // todo
        // make validation rules as far as possible, based on the key type (incrementing?)
        // or using exists:: ...

        return $rules;
    }