Incrementing child element database fetching in codeigniter - mysql

Question

I'm trying to fetch a series of ids from a database table that includes cross-referencing - each element, a "topic", includes a column for "parent topic" that is within the same table. Given a single parent topic, I want to build an array of all the subtopics that have it as their parent, and then all of the subtopics of those topics, etc.
This doesn't seem like it's that hard, but as a self-taught programmer I feel I'm using all the wrong tools. The merge-array() and var_dump() sections, in particular, feel wrong and I'm not sure about the overall approach. What should I replace these elements with?
function get_subtopics($parent_topic)
{
//returns an array of subtopics minus the first
$all_subs = array();
$query = $this->db->get_where('topics', array('parent_topic' => $parent_topic));
$subs = $query->result_array();
$resubs = array();
$query->free_result();
//push subs to all_subs
//while the subs array has members, find their child
while (count($subs)>0) {
foreach ($subs as $s) {
$query = $this->db->get_where('topics', array('parent_topic' => $s['id']));
$resubs = array_merge($resubs, $query->result_array());
$query->free_result();
}
$all_subs = array_merge($all_subs, $resubs);
var_dump($resubs);
}
//Returns an array of ids
return $all_subs;
}
EDIT:
The objective of this is to form a "pool" of topics from which problems will be drawn for a random generator - I'm trying to get all of the subtopics into one array, with no tree structure to differentiate them. Users that specify a parent topic, like "math" should get an even mix of math subtopics like "algebra", "algebra:quadratics" or "calculus" from which problems will be drawn. Hope that clarifies a little.

Answer 1

There are 2 ways to do this either just get all the records from the database and build a tree structure using a php recursive function like below.
//Build menu array containing links and subs
$items = Array(
//highest level
'cms' => Array(
'title' => 'CMS',
//Array containing submenu items for cms
'subs' => Array(
'intro-to-cms' => Array('title' => 'Intro to CMS'),
'specific-cms' => Array('title' => 'Specific CMS'),
'installing-a-cms' => Array('title' => 'Installing a CMS')
),
)
);
//Display the menu
echo navlinks($items, $page);
/**
* Recursive function creates a navigation out of an array with n level children
* #param type $items
* #return string containing treestructure
*/
function navlinks($items, $page=false)
{
$html = '<ul>';
foreach ($items AS $uri => $info) {
//Check if the pagename is the same as the link name and set it to current when it is
$html .= '<li'.($info['title'] == $page ? ' class="current"' : '').'>';
echo ' ' . $info['title'] . '';
//If the link has a sub array, recurse this function to build another list in this listitem
if (isset($info['subs']) && is_array($info['subs'])) {
$html .= navlinks($info['subs']);
}
$html .= '</li>';
}
$html .= '</ul>';
return $html;
}
In order to just filter on 1 parent with its underlying children you will need a rather tricky query in advance like explained in a previous comment on stackoverflow. (link below)
MySQL parent -> child query