Allow proc as group_method for grouped collection select

Since Rails v5.2, the group method could be a callable object.

CHANGELOG.md

@@ -3,6 +3,7 @@
 * Add support for Rails 7.0 and Ruby 3.1 (no changes required)
 * Fix escaping issue on boolean input with `include_hidden: false` and custom wrapper.
 * Update Bootstrap install generator version 5. [@mhw](https://github.com/mhw)
+* Accept proc as `group_method` for grouped collection select
 
 ## 5.1.0
 

lib/simple_form/inputs/grouped_collection_select_input.rb

@@ -23,7 +23,7 @@ module SimpleForm
 
       # Sample collection
       def collection
-        @collection ||= grouped_collection.map { |collection| collection.try(:send, group_method) }.detect(&:present?) || []
+        @collection ||= grouped_collection.map { |collection| group_method.respond_to?(:call) ? group_method.call(collection) : collection.try(:send, group_method) }.detect(&:present?) || []
       end
 
       def group_method

test/inputs/grouped_collection_select_input_test.rb

@@ -66,6 +66,19 @@ class GroupedCollectionSelectInputTest < ActionView::TestCase
     end
   end
 
+  test 'grouped collection allows overriding group_method using a lambda' do
+    with_input_for @user, :tag_ids, :grouped_select,
+      collection: { Authors: %w[Jose Carlos] },
+      group_method: ->(i) { i.last }
+
+    assert_select 'select.grouped_select#user_tag_ids' do
+      assert_select 'optgroup[label=Authors]' do
+        assert_select 'option[value=Jose]', 'Jose'
+        assert_select 'option[value=Carlos]', 'Carlos'
+      end
+    end
+  end
+
   test 'grouped collection accepts group_label_method option' do
     with_input_for @user, :tag_ids, :grouped_select,
       collection: { %w[Jose Carlos] => 'Authors' },