From 9fdded05c562bfbaea6bf91199c716cb61b9a91d Mon Sep 17 00:00:00 2001
From: Kenta Ishizaki <kentaishizaki@55728.jp>
Date: Mon, 20 Apr 2026 08:13:05 +0900
Subject: [PATCH] Fix label `for` not matching input `id` when collection value
 is nil

When a collection contains a nil value (e.g. `[['Undefined', nil]]`),
the label's `for` attribute gets a trailing underscore
(`for="model_method_"`) while the input's `id` does not
(`id="model_method"`), making the label non-functional.

The root cause is in Rails' `sanitize_attribute_name`, which always
concatenates an underscore separator before the sanitized value
(`"#{sanitized_method_name}_#{sanitized_value(value)}"`), even when
`sanitized_value(nil)` returns an empty string.

Override `sanitize_attribute_name` in `CollectionExtensions` to skip
the trailing underscore when the sanitized value is empty, aligning
the label's `for` with the input's `id`.

Fixes #1840
---
 CHANGELOG.md                                  |  1 +
 lib/simple_form/tags.rb                       | 14 +++++++++++++
 .../collection_check_boxes_input_test.rb      | 20 +++++++++++++++++++
 .../collection_radio_buttons_input_test.rb    | 20 +++++++++++++++++++
 4 files changed, 55 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index bc675d2e9..9bd5be39f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 * Ruby 4.0 support (no changes required)
 * Support procs on validators for minlength/maxlength, and improve validators logic across the board to match Rails [#1859](https://github.com/heartcombo/simple_form/pull/1859)
+* Fix label `for` attribute not matching input `id` when collection contains a nil value [#1840](https://github.com/heartcombo/simple_form/issues/1840)
 
 ## 5.4.0
 
diff --git a/lib/simple_form/tags.rb b/lib/simple_form/tags.rb
index 2d779b0dd..fed27ba06 100644
--- a/lib/simple_form/tags.rb
+++ b/lib/simple_form/tags.rb
@@ -26,6 +26,20 @@ def render_collection
         end.join.html_safe
       end
 
+      # Override Rails' sanitize_attribute_name to avoid generating a trailing
+      # underscore when the collection value is nil (e.g. "method_" instead of
+      # "method"), which causes the label's `for` attribute to not match the
+      # input's `id`. See https://github.com/heartcombo/simple_form/issues/1840
+      def sanitize_attribute_name(value)
+        sanitized = sanitized_value(value)
+
+        if sanitized.empty?
+          sanitized_method_name.dup
+        else
+          "#{sanitized_method_name}_#{sanitized}"
+        end
+      end
+
       def wrap_rendered_collection(collection)
         wrapper_tag = @options[:collection_wrapper_tag]
 
diff --git a/test/inputs/collection_check_boxes_input_test.rb b/test/inputs/collection_check_boxes_input_test.rb
index 139ac33fa..03d0e274e 100644
--- a/test/inputs/collection_check_boxes_input_test.rb
+++ b/test/inputs/collection_check_boxes_input_test.rb
@@ -314,4 +314,24 @@ class CollectionCheckBoxesInputTest < ActionView::TestCase
       assert_select 'label.beautiful-label', count: 2
     end
   end
+
+  test 'input check boxes label for attribute matches input id when collection contains nil value' do
+    with_input_for @user, :name, :check_boxes, collection: [['Yes', true], ['Undefined', nil]]
+
+    assert_select 'input[type=checkbox][value=true]#user_name_true'
+    assert_select 'label.collection_check_boxes[for=user_name_true]', 'Yes'
+    assert_select 'input[type=checkbox]#user_name'
+    assert_select 'label.collection_check_boxes[for=user_name]', 'Undefined'
+    assert_no_select 'label[for=user_name_]'
+  end
+
+  test 'input check boxes with nested style label for attribute matches input id when collection contains nil value' do
+    swap SimpleForm, boolean_style: :nested do
+      with_input_for @user, :name, :check_boxes, collection: [['Yes', true], ['Undefined', nil]]
+
+      assert_select 'label[for=user_name_true]'
+      assert_select 'label[for=user_name]'
+      assert_no_select 'label[for=user_name_]'
+    end
+  end
 end
diff --git a/test/inputs/collection_radio_buttons_input_test.rb b/test/inputs/collection_radio_buttons_input_test.rb
index 94c5aaaa5..2b75fcb6a 100644
--- a/test/inputs/collection_radio_buttons_input_test.rb
+++ b/test/inputs/collection_radio_buttons_input_test.rb
@@ -437,4 +437,24 @@ class CollectionRadioButtonsInputTest < ActionView::TestCase
       assert_select 'label.beautiful-label', count: 2
     end
   end
+
+  test 'input radio label for attribute matches input id when collection contains nil value' do
+    with_input_for @user, :name, :radio_buttons, collection: [['Yes', true], ['Undefined', nil]]
+
+    assert_select 'input[type=radio][value=true]#user_name_true'
+    assert_select 'label.collection_radio_buttons[for=user_name_true]', 'Yes'
+    assert_select 'input[type=radio]#user_name'
+    assert_select 'label.collection_radio_buttons[for=user_name]', 'Undefined'
+    assert_no_select 'label[for=user_name_]'
+  end
+
+  test 'input radio with nested style label for attribute matches input id when collection contains nil value' do
+    swap SimpleForm, boolean_style: :nested do
+      with_input_for @user, :name, :radio_buttons, collection: [['Yes', true], ['Undefined', nil]]
+
+      assert_select 'label[for=user_name_true]'
+      assert_select 'label[for=user_name]'
+      assert_no_select 'label[for=user_name_]'
+    end
+  end
 end