PHP: Using '$this' within a '$this'? - php

Question

Probably a bit of a newbie question, but I'm self-taught, and I'm trying to edit some code that is out of my comfort zone, and coming unstuck. Please help!
It is a function that selects data from a MySQL database and returns an array. It uses a couple of other 'core' functions to help with the DB connections.
The original script looks like this:
class Core{
protected $db, $result;
private $rows;
public function __construct() {
$this->db = new mysqli('localhost', 'root', 'password', 'db');
}
public function query($sql){
$this->result = $this->db->query($sql);
}
public function rows(){
for($x = 1; $x <= $this->db->affected_rows; $x++){
$this->rows[] = $this->result->fetch_assoc();
}
return $this->rows;
}
}
class Chat extends Core{
public function fetchMessages(){
$this->query("
SELECT `chat`.`message`,
`users`.`username`,
`users`.`user_id`
FROM `chat`
JOIN `users`
ON `chat`.`user_id` = `users`.`user_id`
ORDER BY `chat`.`timestamp`
DESC
");
return $this->rows();
}
}
The problem is, within the fetchMessages() function in the Chat class there is a SELECT clause which JOINS the "chat" table with the "users" table to fetch the username. If, for whatever the reason, (deleted, banned, quit, etc), the user ID doesn't exist in the user table, the SELECT clause returns no results.
In order for it still to return the message if the user doesn't exist, I think I need to seperate the JOIN into 2 SELECT CLAUSES:
First, SELECT message, user_id FROM chat ORDER BY timestamp DESC;
Then, SELECT username FROM users WHERE user_id = $user_id and RETURN "Guest" if no rows are found.
(I have got the pseudo-code or logic right in my head, I just can't code it!)
My problem is, because I am using the $this-> notation, I don't know how to include a second instance inside the function. What I want to do is something like this:
public function fetchMessages(){
$this->query("
SELECT `message`, `user_id` FROM `chat` ORDER BY `chat`.`timestamp` DESC
");
$rows = $this->rows();
foreach ($rows as $row) {
$uid = $row['user_id'];
$this[2]->query("
SELECT `username` FROM `users` WHERE `user_id` = `$uid`;
");
$user = $this[2]->rows();
if ( $user['username'] == "" ) {
$username = "Guest";
} else {
$username = $user['username'];
}
$return_array[] = array($row['message'],$username);
}
return $return_array;
}
Can anyone understand what I am trying to do, and re-write my pseudo-code so that it doesn't use two '$this->'s and actually works?
I would really appreciate any help...

Answer 1

You simply need to change JOIN users to LEFT JOIN users.
Your current code performs an INNER JOIN, which produces result rows only when there are rows to join on both tables. A LEFT JOIN will produce results for all rows that exist on the left-hand-side table, substituting NULL for the values from the right-hand-side table when no corresponding rows exist there.
In your case, this means that you will be getting back rows for all messages, even if there is no corresponding user for some of them.
See also a visual explanation of SQL joins.