Example: If x-db-default-expression is present then ensure it is present as `\yii\db\Expression(...)` in model validation `rules()`. ``` x-db-default-expression x-db-default ``` Pulled from https://github.com/cebe/yii2-openapi/issues/122 Also part of https://github.com/php-openapi/yii2-openapi/issues/10
