SilverStripe GroupedList with has_one data - silverstripe

Question

I'm banging my head trying to figure this out and I feel like I'm over thinking it.
So I've built 2 DataObjects (Brand, Product) and 1 Controller/Page (ProductType).
ProductType has many Products
Product has one Brand
Product has one ProductType
Brand has many Products
What I'm looking to do is:
ProductType pages should make available a list of Brands that are used ONLY by the Products of that Type.
Now I have one somewhat working but it feels hacky and not very portable. Example below:
ProductType Controller:
public function getBrands() {
if($products = $this->Products()) {
$group = GroupedList::create($products->sort('BrandID'))->GroupedBy('BrandID');
}
}
Product Type Template:
<% if Brands %>
<ul>
<li>All</li>
<% loop Brands %>
<% loop Children.Limit(1) %>
<li>$Brand.Title</li>
<% end_loop %>
<% end_loop %>
</ul>
<% end_if %>
Is there a way I can build a method in the ProductType controller that would return a DataList of just the Brands used by Products of that type?
Using Silverstripe 3.1.3
Let me know if I need to be more clear about something and thanks!

Answer 1

Ah, i guess you had a misunderstanding of a GroupedList
You already have the Datalist of all products of this product type.
It's the has_many relation
$this->Products()
now you want to group the current products by BrandID. Your method naming might be confusing, so let's rename it a bit:
public function getProductsGroupedByBrands() {
if($products = $this->Products()) {
$group = GroupedList::create($products->sort('BrandID'))
->GroupedBy('BrandID');
}
}
So in your template you have a GroupedList you can loop over.
<% if Products %>
<ul>
<li>All</li>
<% loop ProductsGroupedByBrands %>
<li>
<%-- here you should be able to see the grouping relation --%>
$Brand.Title
<%-- in doubt use the First element to get the current Brand, it's a has_one -->
<% with $Children.First %>
$Brand.Title
<% end_with %>
<ul>
<% loop Children %>
<%-- here are the products grouped by brand --%>
<li>$Title</li>
<% end_loop %>
</ul>
</li>
<% end_loop %>
</ul>
<% end_if %>
See also Silverstripe Docs or Grouping Lists