Q:

Grouping a select list (optgroup) in CakePHP 3

I am trying to make a list of grouped things in CakePHP 3, to create a grouped list of things in a select list in a form. I'm not sure if I am missing something or if I'm expecting too much of Cake and should be doing more myself.
I have a controller called Issues and a self-referencing column called RelatedIssues.  Each Issue belongs to a System, and it's the systems I want the issues grouped by.
In my IssuesTable.php:
$this->belongsTo('RelatedIssues', [
    'className' => 'Issues',
    'foreignKey' => 'issue_id'
]);

$this->belongsTo('Systems', [
    'foreignKey' => 'system_id',
    'joinType' => 'INNER'
]);

...and in my IssuesController's edit method:
$relatedIssues = $this->Issues->RelatedIssues->find('list', [
    'groupField' => 'system_id'
]);

When I get to the drop-down list, items are grouped by system_id as specified, but I cannot figure out how to get them grouped by the System's title field.  Is this even possible, or do I have to write a nice nested foreach structure to do this myself?

A:

should be (can'try it now):
$relatedIssues = $this->Issues->RelatedIssues->find('list', [
    'groupField' => 'system.title'
])->contain('Systems');

