Use `lookup_model_names` instead `object_name`.

b5f2ea420d (commitcomment-1959017)

lib/simple_form/inputs/base.rb

@@ -83,7 +83,7 @@ module SimpleForm
       end
 
       def input_class
-        "#{object_name}_#{reflection_or_attribute_name}"
+        "#{lookup_model_names.join("_")}_#{reflection_or_attribute_name}"
       end
 
       private

test/form_builder/wrapper_test.rb

@@ -16,6 +16,17 @@ class WrapperTest < ActionView::TestCase
     assert_select 'div.user_name'
   end
 
+  test 'wrapper should add the attribute name class for nested forms' do
+    @user.company = Company.new(1, 'Empresa')
+    with_concat_form_for @user do |f|
+      concat(f.simple_fields_for(:company) do |company_form|
+        concat(company_form.input :name)
+      end)
+    end
+
+    assert_select 'div.user_company_name'
+  end
+
   test 'wrapper should add the association name class' do
     with_form_for @user, :company
     assert_select 'div.user_company'