Rails Simple-Form group_method - ruby-on-rails

Question

I came across the following on the simple_form github repo:
f.input :country_id, :collection => #continents, :as => :grouped_select, :group_method => :countries
The thing that caught my attention was the :group_method wich would be exceptionally usefull when creating a selectbox that gives options based on what's in the database. The only thing I can't work out is what kind of input the :group_method expects, and where to put the method.
For instance, I want to create a selectbox for the table column :product_type. I imagine I would write something like this in my simple form:
= f.input :product_type_contains, :collection => #products, :as => :grouped_select, :group_method => :product_types
where :product_type would be the method that is being called. But I don't know what kind of method I should write, what kind of result simple_form expects, and if I should put it in the Product class, Product.rb. Any help would be greatly appreciated!

Answer 1

According to the test suite, simple_form seems to expect the type of arrays or hashes that you would use with grouped_options_for_select:
test 'grouped collection accepts group_label_method option' do
with_input_for #user, :tag_ids, :grouped_select,
:collection => { ['Jose', 'Carlos'] => 'Authors' },
:group_method => :first,
:group_label_method => :last
[...]
test 'grouped collection accepts label and value methods options' do
with_input_for #user, :tag_ids, :grouped_select,
:collection => { 'Authors' => ['Jose', 'Carlos'] },
:group_method => :last,
:label_method => :upcase,
:value_method => :downcase
[...]
Presumably, you could write a class method on Product.rb that creates a structure similar, or even try using grouped_options_for_select(#products)...
Hope this gets you on the right path.