private function getInsertFieldsString()
    {
        // Get field keys
        $fieldKeys = array_keys($this->fields);

        // Create placeholders
        $fieldsPlaceholders = array_map(function ($field) {
            return ':' . $field;
        }, $fieldKeys);

        // Return complete fields string
        return '(' . implode(',', $fieldKeys) . ') VALUES ('. implode(',', $fieldsPlaceholders) . ')';
    }