Updating database values each time php method is called - php

Question

I have a php class
<?php
class Students
{
public $sesscode;
public $db_data;
function __construct($sesscode)
{
$this->sesscode = $sesscode;
$this->db_data = dbfetch(null, "SELECT * FROM students WHERE code = ?", [$this->sesscode]); //this simply returns data from db
$this->db_data = (empty($this->db_data)) ? $this->createProfile() : $this->db_data[0];
}
public function createProfile()
{
dbquery(null, "INSERT INTO students (code) VALUES (?)", [$this->sesscode]);
return dbfetch(null, "SELECT * FROM students WHERE code = ?", [$this->sesscode])[0];
}
public function updateAccount($row, $key, $val)
{
$data = json_decode($this->db_data[$row], true);
if (!$data) {
$data = [
$key => $val,
];
} else {
$data[$key] = trim($val);
}
dbquery(null, "UPDATE students SET account = ? WHERE code = ?", [json_encode($data), $this->sesscode]);
}
public function getData($row)
{
return json_decode($this->db_data[$row], true);
}
}
?>
Usage:
$student = new Student($sesscode); //sesscode is just some string
if (!empty($_POST)){
$student->updateAccount("account", "course", $_POST['course']);
//see class code/updateAccount method above for parameters. This means update dB table row "account", decode json and set course to post value of course
$student->updateAccount("account", "email", $_POST['email']);
$student->updateAccount("account", "phone", $_POST['phone']);
$student->updateAccount("account", "institution", $_POST['institution']);
}
My problem is, only the last line executes (and no, it isn't overwriting the data)
Moving the line that sets phone to take the place of institution will set phone and ignore those above it.

Answer 1

The line
$data = json_decode($this->db_data[$row], true);
resets the value of $data every time your updateAccount account function runs. It resets it to the value you fetched from the database when you created the object. Therefore this doesn't take account of any changes you've made in previous calls to updateAccount. So in fact it does overwrite your data, but probably just quite not in the way you imagined.
You need to either:
a) keep $data in-memory as a (private) property of the class rather than resetting it each time updateAccount runs,
or
b) re-fetch the existing data from the database each time updateAccount runs,
or
c) Store your data in relational format (with fields in separate columns) instead of as JSON within a single column - meaning you could update individual fields without interfering with the others,
and/or
d) provide a version of updateAccount where multiple values can be updated simultaneously - running separate UPDATE queries for each field is very inefficient.