I currently have designed a structure where a custom Page has a child DataObject (Topic) which houses several other DataObjects (EducationPost). I am trying to do a couple things.

1. I have built a listbox field that I want to have manage the many_many relation between Topic and EducationPost, but I am not sure how to get the field to work with that table in the database when I put it into the CMSFields for EducationPost.

2. I need to access my EducationPost objects from the Page that houses the Topic. I attempted to do this (from the template) by using $Topic.EducationPost but I am not certain that will work (cannot test until the table is properly managed in the above step).

Any help that can be provided is appreciated.

Prerequisites:

class EducationPost extends DataObject {
	private static $db = ['Title' => 'Varchar'];
	private static $belongs_many_many = ['Topics' => 'Topic'];
}
class Topic extends DataObject {
	private static $db = ['Title' => 'Varchar'];
	private static $many_many = ['EducationPosts' => 'EducationPost'];
	private static $has_many = ['Pages' => 'Page'];
}
class Page extends SiteTree {
	private static $db = ['Title' => 'Varchar'];
	private static $has_one = ['Topic' => 'Topic'];
}

1. ... but I am not sure how to get the field to work with that table in the database when I put it into the CMSFields for EducationPost.

class EducationPost extends DataObject {
	// ...

	public function getCMSFields() {
		$fields = parent::getCMSFields();

		$source = Topic::get()->map('ID', 'Title')->toArray();
		$value = array_keys($this->Topics()->getIDList());
		$fields->addFieldToTab('Root.Main', 
			ListboxField::create('Topics', 'Topic', $source, $value, $size = null, $multiple = true)
		);

		return $fields;
	}
}

Though, the ListboxField maybe become quite lengthy if there are too many Topics. Maybe you should use a GridField for this instead.

2. ... using $Topic.EducationPost but I am not certain that will work

// Page.ss
$Title // Title of Page
$Topic.Title // Title of Topic
<% loop $Topic.EducationPosts %>
	$Title // Title of EducationPost
<% end_loop %>

// ... or in php
$this->Title; // Title of Page
$this->Topic()->Title; // Title of Topic
foreach ($this->Topic()->EducationPosts() as $EducationPost) {
	$EducationPost->Title; // Title of EducationPost
}

// also vice versa
<% loop $Topic.EducationPosts %>
	$Title // Title of EducationPost
	<% loop $Topics %>
		$Title // Title of Topic
		<% loop $Pages %>
			$Title // Title of Page
		<% end_loop %>
	<% end_loop %>
<% end_loop %>

https://docs.silverstripe.org/en/3.2/developer_guides/model/relations/

Thanks for your help. I got the listbox working but i'm still having some issues with getting the pages to show the posts associated with the topic. Here's what I currently have.

Prerequisites:

class EducationPost extends DataObject {
	private static $db = ['Title' => 'Varchar'];
	private static $belongs_many_many = ['Topics' => 'Topic'];
}
class Topic extends DataObject {
	private static $db = ['Title' => 'Varchar'];
	private static $has_one = ['TopicAggregatorPage' => 'TopicAggregatorPage'];
	private static $many_many = ['EducationPosts' => 'EducationPost'];
}
class TopicAggregatorPage extends EducationPage {
	private static $db = ['Title' => 'Varchar'];
	private static $has_one = ['Topic' => 'Topic'];
}

TopicAggregatorPage:

<div class="row row-eq-height">
	<% if $Topic.EducationPosts %>
		<% loop $Topic.EducationPosts.Sort("Created", DESC) %>
			<% if $MultipleOf(3) %>
				<div class="row row-eq-height">
			<% end_if %>
				<div class="col-xs-12 col-sm-6 well sub-detail-block">
				<img src="$MainImage.Filename">
				<h3>$Title</h3>
				<div class="text">$Teaser</div>
				<p>$Topic</p>
				</div>
			<% if $MultipleOf(2) %>
				<% if not $Last %>
					</div>
				<% end_if %>
			<% end_if %>
		<% end_loop %>
	<% else %>
		<h4>No results to display.</h3>
	<% end_if %>

Currently this only ever shows me "No results to display." despite the fact that I have several posts associated with the page I'm looking at.

Additionally, I have a pagetype EdAggregatorPage that is a parent to several TopicAggregatorPages in the site tree, and I am trying to display posts for those topics on the EdAggregatorPage. I wrote a custom function to get these and build an arraylist (so I can sort, limit, etc as we decide what exactly we want the page to have). That code is as follows:

public function getChildPosts() {
		
	$array = Array();
		
	foreach($Child as $topic){
		
		foreach($topic->EducationPosts as $edPost){
				array_push($array, $edPost);
		}
	}
		
	$list = new ArrayList($array);

	return $list;
}

This worked previously, but has stopped working since I made the updates to fix the listbox. Am I missing something in the names or am I doing something else wrong? I feel like I am close but just can't quite sort it out.

Currently this only ever shows me "No results to display." despite the fact that I have several posts associated with the page I'm looking at.

Works for me. Are you sure your page has a topic? Try a "$TopicID" in your template, it should output some value.

edit: http://www.sspaste.com/paste/show/575d52d6a9c15

As in your getChildPosts() function: your variable "$Child" is undefined and "$topic->EducationPosts" should be a function like "$topic->EducationPosts()"