How to limit the select columns with bindGraph ?

<?php

$graph = '{ "Ancestors": {}, "Descendants": {} }';
$c = $modx->newQuery('myTree');

// This subquery was not accessible in the loop
$c->select('( SELECT COUNT(*) 
	FROM '.$modx->getTableName('myTreeClosure').'
	WHERE ancestor = myTree.id
    AND descendant != myTree.id ) AS `nb_branches`');

$c->where(array(
    'Descendants.ancestor' => 0,
    'Descendants.depth' => 0,
    'myTree.leaf' => $id,
));
$c->groupBy('myTree.title');

$branches = $modx->getCollectionGraph( 'myTree', $graph, $c );

foreach ($branches as $branch) {
	//That was not working even though the SQL dump was working correctly
	$nb = $branch->get('nb_branches');
}

<?php
// Give a used prefix to the alias of the retreived field - here i use myTree
$c->select('( SELECT COUNT(*) 
	FROM '.$modx->getTableName('myTreeClosure').'
	WHERE ancestor = myTree.id
    AND descendant != myTree.id ) AS `myTree_nb_branches`');
	
//Then the loop was working 
foreach ($branches as $branch) {
	$nb = $branch->get('nb_branches');
	//or
	$array[] = $branch->toArray();
}

<?php
$graph = '{ "Relate": {}, "RelateToo": {} }';

$c = $modx->newQuery('myTable');
$c->bindGraph( $graph );

//Instead of having all the columns from the Related tables i want to select manually which columns will be retreived
$c->select( /* getSelectColumns does not work fror the graph ?  */ ); 

if ($c->prepare() && $c->stmt->execute()) {
	while ($result = $c->stmt->fetch(PDO::FETCH_ASSOC)) {
		//Loop
	}
}