Table joins accross multiple database tables and objects in PHP and MySQL - php

Question

I have 4 classes, each with their own database tables:
GlobalObject
Image
Project
Page
GlobalObject has 3 properties
global_object_id
added_by
datetime_added
Image, Project and Page have many different fields, they ALL have the following though:
id
global_object_id (Foreign key)
At the moment, my class structure has Images, Projects and Pages as subclasses of the GlobalObject class, this allows the 3 subclasses access to the variables that are required of them, datetime_added etc.
How should I set the properties using PHP and MySQL? At the moment, ALL of the fields (including those in the global_object table) are in each section's table (except global_object_id which cannot exist without the new table - and is why I need to do this), So i request the data from the Image table for example, which includes all the properties in the parent class and is set using parent::set_by_row($database_args), and $this->set_by_row($database_args);
At the moment I do the following (Using Image as an example):
class Image extends GlobalObject
{
$id;
$title;
function set_by_id($id)
{
// Select Q from Image table
$this->set_by_row($db_result);
}
function set_by_row($args)
{
parent::set_by_row($args);
// Set class properties
}
}
To reiterate: I want to be able to do the following:
$image = new Image()
$image->set_by_global_object_id(23);
And the image be set, including everything in the parent class (which is stored in a separate table)
Please ask away if any of that is unclear.

Answer 1

A very convenient way is offered by those "magic methods", like __get and __set. The following example provides read-only access to the data, but can easily be extended with a __set method.
abstract class GlobalObject {
protected $_data = array();
/* Magic Method: Enable read access via $this->name */
public function __get($key) {
if (isset($this->_data[$key])) {
return $this->_data[$key];
}
throw new Exception('Property "'. $key .'" not found.');
}
protected function _setFromArray(array $data) {
// .. do whatever needs to be done with the data ..
$this->_data = $data;
}
}
Now just extend this class:
class Image extends GlobalObject {
public function setByGlobalId($id) {
$result = mysql_query('SELECT i.*, g.* FROM images AS i
LEFT JOIN global_objects AS g
ON g.global_object_id = i.global_object_id
WHERE i.global_object_id = '. intval($id) .' LIMIT 1');
// #todo check if result is empty
$data = mysql_fetch_assoc($result);
$this->_setFromArray($data);
}
}
And a simple example:
$image = new Image();
$image->SetByGlobalId(42);
echo $image->added_by;