动态设置更新语句不起作用

Question

its shows that syntax error in sql statement with this message database query failed : ... use right syntax near " at line 1 however i can't find one plz help

public function update(){
    global $database;
    $attributes = $this->attributes();
    $attribute_pairs = array();
    foreach ($attributes as $key => $value) {
        # code...
        $attribute_pairs[] = "$key = $value";
    }

    $sql = "UPDATE ".self::$table_name." SET ";
    $sql .= join(", ", $attribute_pairs);
    $sql .= " WHERE id=".$this->id;
    $database->query($sql);
    return ($database->affected_rows() == 1) ? true : false;
}