class Upsert

class Upsert extends QueryUpsert {
    
    /**
     * {@inheritdoc}
     */
    public function __toString() {
        // Create a sanitized comment string to prepend to the query.
        $comments = $this->connection
            ->makeComment($this->comments);
        // Default fields are always placed first for consistency.
        $insert_fields = array_merge($this->defaultFields, $this->insertFields);
        $insert_fields = array_map(function ($field) {
            return $this->connection
                ->escapeField($field);
        }, $insert_fields);
        $query = $comments . 'INSERT INTO {' . $this->table . '} (' . implode(', ', $insert_fields) . ') VALUES ';
        $values = $this->getInsertPlaceholderFragment($this->insertValues, $this->defaultFields);
        $query .= implode(', ', $values);
        // Updating the unique / primary key is not necessary.
        unset($insert_fields[$this->key]);
        $update = [];
        foreach ($insert_fields as $field) {
            $update[] = "{$field} = VALUES({$field})";
        }
        $query .= ' ON DUPLICATE KEY UPDATE ' . implode(', ', $update);
        return $query;
    }

}