Laravel add relation objects to new eloquent model

I wanna use a form partial for a new and for an existing object.

I found similar questions:

• Create new eloquent instance with relationships
• Laravel getting builder instead of relation

but I can't and don't wanna save the parent object.

I came from the rails view with active record. I can do the following:

Let's say I have a category which includes many products:

category = Category.new
category.products << Product.new

Now I can iterate through the products like

category.products.each do ...

Now I want the same in laravel with an eloquent model

$category = new Category();
$category->products()->....

add does not exist for the Builder. save needs a stored category attach needs the same

is there a way to get my idea working? My goal is to use the same form partial for edit and creating a model with defined relations.