Use the first non-empty object to detect the label and value method for

grouped select

Fixes #978

CHANGELOG.md

@@ -21,6 +21,7 @@
   * Collection input that uses automatic collection translation properly sets checked values.
   Closes [#971](https://github.com/plataformatec/simple_form/issues/971) [@nashby](https://github.com/nashby)
   * Collection input generates `required` attribute if it has `prompt` option. [@nashby](https://github.com/nashby)
+  * Grouped collection uses the first non-empty object to detect label and value methods.
 
 ## deprecation
   * Methods on custom inputs now accept a required argument with the wrapper options.

lib/simple_form/inputs/grouped_collection_select_input.rb

@@ -22,7 +22,7 @@ module SimpleForm
 
       # Sample collection
       def collection
-        @collection ||= grouped_collection.first.try(:send, group_method) || []
+        @collection ||= grouped_collection.map { |collection| collection.try(:send, group_method) }.detect(&:present?) || []
       end
 
       def group_method

test/inputs/grouped_collection_select_input_test.rb

@@ -97,6 +97,24 @@ class GroupedCollectionSelectInputTest < ActionView::TestCase
     end
   end
 
+  test 'grouped collections finds the default label method from the first non-empty object' do
+    Agent = Struct.new(:id, :name)
+    agents = [["First", []], ["Second", [Agent.new(7, 'Bond'), Agent.new(47, 'Hitman')]]]
+
+    with_input_for @user, :tag_ids, :grouped_select,
+      collection: agents,
+      group_label_method: :first,
+      group_method: :last,
+      include_blank: false
+
+    assert_select 'select.grouped_select#user_tag_ids' do
+      assert_select 'optgroup[label=Second]' do
+        assert_select 'option[value=7]', 'Bond'
+        assert_select 'option[value=47]', 'Hitman'
+      end
+    end
+  end
+
   test 'grouped collection accepts label and value methods options' do
     with_input_for @user, :tag_ids, :grouped_select,
       collection: { 'Authors' => ['Jose', 'Carlos'] },