public function primaryKey()
    {
        if (empty($this->_context['schema']['_constraints']) ||
            !is_array($this->_context['schema']['_constraints'])
        ) {
            return [];
        }
        foreach ($this->_context['schema']['_constraints'] as $data) {
            if (isset($data['type']) && $data['type'] === 'primary') {
                return isset($data['columns']) ? (array)$data['columns'] : [];
            }
        }

        return [];
    }