October CMS - deferred bindings with images that aren't attachments - forms

Question

I'm using the October CMS and I'm having some trouble with deferred bindings.
I have two tables: products and product_images. I've split up my backend form into two tabs, one for product details and one for the product images:
I have my relationships set up correctly and use the following code (placed in a partial) to render the product images list:
<?= $this->relationRender('product_images'); ?>
The images tab looks like this:
The problem happens when I try to create a new image. When saving the image from the image modal, I get this exception:
I understand why there would be a constraint violation: The main record hasn't been saved yet, so there's no id for the image record to reference. In other words, the product image can't be associated with the product because the product doesn't exist yet.
The OctoberCMS documentation on deferred binding hints at a solution. But the documentation also states,
Deferred bindings are supported in the back-end Form behavior
automatically
Indeed, I haven't explicitly written any back-end form processing code. So even if I wanted to follow the instructions on deferred bindings, I wouldn't know where to put it. Any suggestions?
UPDATE:
In my config_relations.yaml file, I've set deferredBinding to true, but it made no difference:
product_images:
label: Image
deferredBinding: true
My products controller looks like:
class Products extends \Backend\Classes\Controller
{
public $implement = [
'Backend.Behaviors.FormController',
'Backend.Behaviors.ListController',
'Backend.Behaviors.RelationController'
];
public $formConfig = 'config_form.yaml';
public $listConfig = 'config_list.yaml';
public $relationConfig = 'config_relation.yaml';
public function __construct()
{
parent::__construct();
BackendMenu::setContext('MyPlugin.Products', 'products');
}
public function index()
{
$this->makeLists();
$this->makeView('index');
}
I do not have a product_images controller. I'm not sure why. Is that the issue?

Answer 1

My mistake was that I had put constraints on the product_id column in the product_images table:
$table->integer('product_id')->unsigned();
$table->foreign('product_id')->references('id')->on('me_myplugin_products');
Apparently I need to allow that column to be null. Changing it to this worked:
$table->integer('product_id')->nullable();