CMSGetRelationChainExtension: get relation chains for new records

CMSGetRelationChainExtension.php

<?php
/**
 * get relation chains for new records
 */
class CMSGetRelationChainExtension extends Extension {

	private $parts = null;

	private $keys = array(
		'field',
		'relation',
		'item',
		'id',
		'Item',
	);

	/**
	 * Get all parts from the CMS url querystring variable as associative arrays
	 */
	private function CMSGetParts(){
		if (is_array($this->parts)) {
			return $this->parts;
		}

		$parts = explode('EditForm/', $_GET['url']);
		array_shift($parts);
		
		$parts_array = array();
		$key_count   = count($this->keys);
		foreach ($parts as $index => $part) {
			$values = explode('/',$part);
			$a = array();
			if (count($values) == $key_count) {
				$a = array_combine($this->keys, $values);
				$parent = end($parts_array);
				// Debug::show($parent);
				if (!$parent || !isset($parent['class'])) {
					// First relation is a class name (always?)
					$a['class'] = $a['relation'];
				}
				else {
					// Get class from parent's relation
					$a['class'] = singleton($parent['class'])->getRelationClass($a['relation']);

				}
			}
			$parts_array[] = $a;
		}

		$this->parts = $parts_array;
		// Debug::show($this->parts);
		return $this->parts;
	}

	/**
	 * Get part of the relation chain based off index
	 * Supports negative indexes. (ie. parent = -2)
	 * @param integer $index
	 */
	public function CMSGetPartByIndex(integer $index) {
		// Ensure parts are stored
		$parts = $this->CMSGetParts();

		// get correct array index
		if ($index < 0 ) {
			// think this is right
			if (abs($index) > count($parts)) {
				return null;
				// user_error(sprintf("CMSGetPartByIndex %d can't be lower than %d", $index, count($parts)));
			}
			$index = count($parts) + $index; 
		}


		$part = array();
		if ($index >= 0 && $index < count($parts)) {
			return $parts[$index];
		}
		else {
			// to pass /dev/build this has to return null
			return null;
			// user_error("CMSGetPartByIndex - Index out of range: " . $index);
		}
	}

	public function CMSPart2Object($p) {

		// when null is passed in because the object doesn't exist
		if (!is_array($p)) {
			return null;
		}

		// when submitting a new / edit action instead of ID
		if (!is_numeric($p['id'])) {
			return null;
		}

		$Object = DataObject::get_by_id( $p['class'], $p['id']);
		return $Object;
	}

	/**
	 * Get part of the relation chain based on relation or id
	 * TODO: problems if relation type is used multiple times in the chain
	 * @param [type] $key   relation / id
	 * @param [type] $value value of key
	 */
	public function CMSGetPartByKey($key, $value) {
		// Ensure parts are stored
		$parts = $this->CMSGetParts();

		if (!in_array($key, $this->keys)) {
			user_error("CMSGetPartByIndex - key not found:" . $key);	
		}

		foreach (array_reverse($parts) as $part) {
			$p = array();
			if ($part[$key] == $value) {
				return $part;
			}
		}
		return null; // to pass /dev/build this has to return null

		// user_error("CMSGetPartByIndex - value not found: " . $value);
	}
}

example.php

<?php
// This code is run on Schedule which has_one CourseDate (CourseDate has_many Schedules)
public function getCMSFields() {
	$fields = parent::getCMSFields();

	// populate default values 
	if(!$this->ID) {
		$parent = $this->CMSGetPartByIndex(-2);
	 	$CourseDate = $this->CMSPart2Object($parent);

		if ( $CourseDate ) {
	    	foreach (array('StartDate', 'EndDate') as $property) {
	    		if (empty($this->$property)) {
					$this->$property = $CourseDate->$property;
				}
	    	}
	    }
	}

	return $fields;
}