Declare ActionView::Helpers::FormBuilder#id

`ActionView::Helpers::FormBuilder#id`
---

Generate an HTML `id` attribute value.

Return the [`<form>` element's][mdn-form] `id` attribute.

```html+erb
<%= form_for @post do |f| %>
  <%# ... %>

  <% content_for :sticky_footer do %>
    <%= form.button(form: f.id) %>
  <% end %>
<% end %>
```

In the example above, the `:sticky_footer` content area will exist
outside of the `<form>` element. [By declaring the `form` HTML
attribute][mdn-button-attr-form], we hint to the browser that the
generated `<button>` element should be treated as the `<form>` element's
submit button, regardless of where it exists in the DOM.

[A similar pattern could be used for `<input>`
elements][mdn-input-attr-form] (or other form controls) that do not
descend from the `<form>` element.

`ActionView::Helpers::FormBuilder#field_id`
---

Generate an HTML <tt>id</tt> attribute value for the given field

Return the value generated by the <tt>FormBuilder</tt> for the given
attribute name.

```html+erb
<%= form_for @post do |f| %>
  <%= f.label :title %>
  <%= f.text_field :title, aria: { describedby: form.field_id(:title, :error) } %>
  <span id="<%= f.field_id(:title, :error) %>">is blank</span>
<% end %>
```

In the example above, the <tt><input type="text"></tt> element built by
the call to <tt>FormBuilder#text_field</tt> declares an
<tt>aria-describedby</tt> attribute referencing the <tt><span></tt>
element, sharing a common <tt>id</tt> root (<tt>post_title</tt>, in this
case).

This method is powered by the `field_id` helper declared in
`action_view/helpers/form_tag_helper`, which is made available for
general template calls, separate from a `FormBuilder` instance.

[mdn-form]: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/form
[mdn-button-attr-form]: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/button#attr-form
[mdn-input-attr-form]: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input#attr-form
[mdn-aria-describedby]: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_aria-describedby_attribute
[w3c-wai]: https://www.w3.org/WAI/tutorials/forms/notifications/#listing-errors

actionview/CHANGELOG.md

@@ -1,4 +1,30 @@
-*   Transforms a Hash into HTML Attributes, ready to be interpolated into ERB.
+*   `ActionView::Helpers::FormBuilder#id` returns the value
+    of the `<form>` element's `id` attribute. With a `method` argument, returns
+    the `id` attribute for a form field with that name.
+
+        <%= form_for @post do |f| %>
+          <%# ... %>
+
+          <% content_for :sticky_footer do %>
+            <%= form.button(form: f.id) %>
+          <% end %>
+        <% end %>
+
+    *Sean Doyle*
+
+*   `ActionView::Helpers::FormBuilder#field_id` returns the value generated by
+    the FormBuilder for the given attribute name.
+
+        <%= form_for @post do |f| %>
+          <%= f.label :title %>
+          <%= f.text_field :title, aria: { describedby: f.field_id(:title, :error) } %>
+          <%= tag.span("is blank", id: f.field_id(:title, :error) %>
+        <% end %>
+
+    *Sean Doyle*
+
+*   Add `tag.attributes` to transform a Hash into HTML Attributes, ready to be
+    interpolated into ERB.
 
         <input <%= tag.attributes(type: :text, aria: { label: "Search" }) %> >
         # => <input type="text" aria-label="Search">

actionview/lib/action_view/helpers/form_helper.rb

@@ -1691,6 +1691,47 @@ def initialize(object_name, object, template, options)
         @index = options[:index] || options[:child_index]
       end
 
+      # Generate an HTML <tt>id</tt> attribute value.
+      #
+      # return the <tt><form></tt> element's <tt>id</tt> attribute.
+      #
+      #   <%= form_for @post do |f| %>
+      #     <%# ... %>
+      #
+      #     <% content_for :sticky_footer do %>
+      #       <%= form.button(form: f.id) %>
+      #     <% end %>
+      #   <% end %>
+      #
+      # In the example above, the <tt>:sticky_footer</tt> content area will
+      # exist outside of the <tt><form></tt> element. By declaring the
+      # <tt>form</tt> HTML attribute, we hint to the browser that the generated
+      # <tt><button></tt> element should be treated as the <tt><form></tt>
+      # element's submit button, regardless of where it exists in the DOM.
+      def id
+        options.dig(:html, :id)
+      end
+
+      # Generate an HTML <tt>id</tt> attribute value for the given field
+      #
+      # Return the value generated by the <tt>FormBuilder</tt> for the given
+      # attribute name.
+      #
+      #   <%= form_for @post do |f| %>
+      #     <%= f.label :title %>
+      #     <%= f.text_field :title, aria: { describedby: f.field_id(:title, :error) } %>
+      #     <%= tag.span("is blank", id: f.field_id(:title, :error) %>
+      #   <% end %>
+      #
+      # In the example above, the <tt><input type="text"></tt> element built by
+      # the call to <tt>FormBuilder#text_field</tt> declares an
+      # <tt>aria-describedby</tt> attribute referencing the <tt><span></tt>
+      # element, sharing a common <tt>id</tt> root (<tt>post_title</tt>, in this
+      # case).
+      def field_id(method, *suffixes, index: @index)
+        @template.field_id(@object || @object_name, method, *suffixes, index: @index)
+      end
+
       ##
       # :method: text_field
       #

actionview/lib/action_view/helpers/form_tag_helper.rb

@@ -79,6 +79,42 @@ def form_tag(url_for_options = {}, options = {}, &block)
         end
       end
 
+      # Generate an HTML <tt>id</tt> attribute value for the given name and
+      # field combination
+      #
+      # Return the value generated by the <tt>FormBuilder</tt> for the given
+      # attribute name.
+      #
+      #   <%= label_tag :post, :title %>
+      #   <%= text_field_tag :post, :title, aria: { describedby: field_id(:post, :title, :error) } %>
+      #   <%= tag.span("is blank", id: field_id(:post, :title, :error) %>
+      #
+      # In the example above, the <tt><input type="text"></tt> element built by
+      # the call to <tt>text_field_tag</tt> declares an
+      # <tt>aria-describedby</tt> attribute referencing the <tt><span></tt>
+      # element, sharing a common <tt>id</tt> root (<tt>post_title</tt>, in this
+      # case).
+      def field_id(object_name, method_name, *suffixes, index: nil)
+        if object_name.respond_to?(:model_name)
+          object_name = object_name.model_name.singular
+        end
+
+        sanitized_object_name = object_name.to_s.gsub(/\]\[|[^-a-zA-Z0-9:.]/, "_").delete_suffix("_")
+
+        sanitized_method_name = method_name.to_s.delete_suffix("?")
+
+        # a little duplication to construct fewer strings
+        if sanitized_object_name.empty?
+          sanitized_method_name.dup
+        elsif suffixes.any?
+          [sanitized_object_name, index, sanitized_method_name, *suffixes].compact.join("_")
+        elsif index
+          "#{sanitized_object_name}_#{index}_#{sanitized_method_name}"
+        else
+          "#{sanitized_object_name}_#{sanitized_method_name}"
+        end
+      end
+
       # Creates a dropdown selection box, or if the <tt>:multiple</tt> option is set to true, a multiple
       # choice selection box.
       #

actionview/lib/action_view/helpers/tags/base.rb

@@ -117,19 +117,7 @@ def tag_name(multiple = false, index = nil)
           end
 
           def tag_id(index = nil)
-            # a little duplication to construct fewer strings
-            case
-            when @object_name.empty?
-              sanitized_method_name.dup
-            when index
-              "#{sanitized_object_name}_#{index}_#{sanitized_method_name}"
-            else
-              "#{sanitized_object_name}_#{sanitized_method_name}"
-            end
-          end
-
-          def sanitized_object_name
-            @sanitized_object_name ||= @object_name.gsub(/\]\[|[^-a-zA-Z0-9:.]/, "_").delete_suffix("_")
+            @template_object.field_id(@object_name, @method_name, index: index)
           end
 
           def sanitized_method_name

actionview/test/template/form_helper_test.rb

@@ -1597,6 +1597,60 @@ def test_form_for_is_not_affected_by_form_with_generates_ids
     ActionView::Helpers::FormHelper.form_with_generates_ids = old_value
   end
 
+  def test_form_for_id
+    form_for(Post.new) do |form|
+      concat form.button(form: form.id)
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      '<button name="button" type="submit" form="new_post">Create Post</button>'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_field_id_with_model
+    value = field_id(Post.new, :title)
+
+    assert_equal "post_title", value
+  end
+
+  def test_field_id_with_predicate_method
+    value = field_id(Post.new, :secret?)
+
+    assert_equal "post_secret", value
+  end
+
+  def test_form_for_field_id
+    form_for(Post.new) do |form|
+      concat form.label(:title)
+      concat form.text_field(:title, aria: { describedby: form.field_id(:title, :error) })
+      concat tag.span("is blank", id: form.field_id(:title, :error))
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      '<label for="post_title">Title</label>' \
+      '<input id="post_title" name="post[title]" type="text" aria-describedby="post_title_error">' \
+      '<span id="post_title_error">is blank</span>'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_field_id_with_index
+    form_for(Post.new, index: 1) do |form|
+      concat form.text_field(:title, aria: { describedby: form.field_id(:title, :error) })
+      concat tag.span("is blank", id: form.field_id(:title, :error))
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      '<input id="post_1_title" name="post[1][title]" type="text" aria-describedby="post_1_title_error">' \
+      '<span id="post_1_title_error">is blank</span>'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_form_for_with_collection_radio_buttons
     post = Post.new
     def post.active; false; end

actionview/test/template/form_tag_helper_test.rb

@@ -187,6 +187,30 @@ def test_form_tag_with_block_and_method_in_erb
     assert_dom_equal expected, output_buffer
   end
 
+  def test_field_id_without_suffixes_or_index
+    value = field_id(:post, :title)
+
+    assert_equal "post_title", value
+  end
+
+  def test_field_id_with_suffixes
+    value = field_id(:post, :title, :error)
+
+    assert_equal "post_title_error", value
+  end
+
+  def test_field_id_with_suffixes_and_index
+    value = field_id(:post, :title, :error, index: 1)
+
+    assert_equal "post_1_title_error", value
+  end
+
+  def test_field_id_with_nested_object_name
+    value = field_id("post[author]", :name)
+
+    assert_equal "post_author_name", value
+  end
+
   def test_hidden_field_tag
     actual = hidden_field_tag "id", 3
     expected = %(<input id="id" name="id" type="hidden" value="3" />)