Answered Why the related object of the parent is not available in the child class?

<?php

class myUser extends modUser {

    public function __construct(xPDO & $xpdo) {
        parent::__construct($xpdo);
        $this->set('class_key', 'myUser');
    }
}

<?php

require_once MODX_CORE_PATH . 'components/mytest/model/mytest/myuser.class.php';

class myChildUser extends myUser {

    public function __construct(xPDO & $xpdo) {
        parent::__construct($xpdo);
        $this->set('class_key', 'myChildUser');
    }
}

<?php

$modx->setLogLevel(xPDO::LOG_LEVEL_INFO);
$modx->setLogTarget(XPDO_CLI_MODE ? 'ECHO' : 'HTML');
$modx->log(xPDO::LOG_LEVEL_INFO, 'Hello!');

$path = MODX_CORE_PATH . 'components/mytest/model/';
$modx->addPackage('mytest', $path, 'mt_');

/* Uncomment line below and comment the next one to make the problem disappear. */
//$user = $modx->newObject('myUser');
$user = $modx->newObject('myChildUser');

$comp = $modx->newObject(
    'myComp',
    array(
        'name' => 'Name'
    )
);
$added = $user->addOne($comp, 'Comp');

return  $added ? 'true' : 'false';

public function __construct(modX &$modx) {
    $this->modx = &$modx;
    $this->modx->addPackage('pkgName', 'path/to/extra/model/', 'tablePrefix_');
    $this->loadBaseClasses();
}

$user = $modx->newObject('myChildUser');

class myChildUser extends myUser {

    public function __construct(xPDO & $xpdo) {
        $xpdo->loadClass('myUser', MODX_CORE_PATH . 'components/mytest/model/mytest/');
        parent::__construct($xpdo);
        $this->set('class_key', 'myChildUser');
    }
}

$modx->loadClass('myUser', MODX_CORE_PATH . 'components/mytest/model/');
$user = $modx->newObject('myChildUser');

class myChildUser extends myUser {
 
    public function __construct(xPDO & $xpdo) {
        $xpdo->loadClass('myUser', MODX_CORE_PATH . 'components/mytest/model/mytest/');
        parent::__construct($xpdo);
        $this->set('class_key', 'myChildUser');
    }
}

class myExtra {
    
    private $modx;
    
    public function __construct(modX $modx) {
        $this->modx = $modx;
        $this->modx->addPackage('mytest', MODX_CORE_PATH . 'components/mytest/model/');
        $this->modx->loadClass('myUser', MODX_CORE_PATH . 'components/mytest/model/mytest/');
    }
}

require_once MODX_CORE_PATH . 'components/mytest/model/mytest/myuser.class.php';

class myChildUser extends myUser {

    public function __construct(xPDO & $xpdo) {
        $xpdo->loadClass('myUser', MODX_CORE_PATH . 'components/mytest/model/mytest/');
        parent::__construct($xpdo);
        $this->set('class_key', 'myChildUser');
    }
}

public function getComposites($className) {
        $composites= array ();
        if ($className= $this->loadClass($className)) {
            if ($ancestry= $this->getAncestry($className)) {
                for ($i= count($ancestry) - 1; $i >= 0; $i--) {
                    $ancestorClass = $this->loadClass($ancestry[$i]);
                    if ($ancestorClass && isset ($this->map[$ancestry[$i]]['composites'])) {
                        $composites= array_merge($composites, $this->map[$ancestry[$i]]['composites']);
                    }
                }
            }
            if ($this->getInherit($className) === 'single') {
                $descendants= $this->getDescendants($className);
                if ($descendants) {
                    foreach ($descendants as $descendant) {
                        $descendantClass= $this->loadClass($descendant);
                        if ($descendantClass && isset($this->map[$descendantClass]['composites'])) {
                            $composites= array_merge($composites, array_diff_key($this->map[$descendantClass]['composites'], $composites));
                        }
                    }
                }
            }
        }
        return $composites;
    }

if (!empty($path) || $ignorePkg) {
    $class= $this->_loadClass($class, $fqn, $included, $path, $transient);
} elseif (isset ($this->packages[$this->package])) {
    $pqn= $this->package . '.' . $fqn;
    // Check depency
    foreach ($this->classMap as $baseClass => $classes) {
        if (array_search($classname, $classes) !== false && !class_exists($baseClass)) { $this->loadClass($baseClass); }
    }
    // End Check
    if (!$pkgClass= $this->_loadClass($class, $pqn, $included, $this->packages[$this->package]['path'], $transient)) {
        foreach ($this->packages as $pkg => $pkgDef) {
            if ($pkg === $this->package) continue;
            $pqn= $pkg . '.' . $fqn;
            if ($pkgClass= $this->_loadClass($class, $pqn, $included, $pkgDef['path'], $transient)) {
                break;
            }
        }
    }
    $class= $pkgClass;
} else {
    $class= false;
}

$vchart = $modx->addPackage('mytest', $modx->getOption('core_path') . 'components/myuser/model/model/', 'test_');
$usr = $modx->newObject('myChildUser');
$usr->set('id', 1);
$comp = $usr->getOne('Comp');
return "<pre>" . print_r($comp->toArray(), true);

foreach ($this->classMap as $baseClass => $classes) {
    if (array_search($classname, $classes) !== false) {
        if ($this->loadClass($baseClass) !== false) {
            break;
        }
    }
}

$a = $modx->newObject('ClassA');

$a = $modx->newObject('ClassA', 'packageA');

class xPDOPackage {
    var $package;
    var $path;

    function getObject() {}
    function getObjectCollection() {}
    function newObject() {}
    ...
}