Introduce field_name view helper

The `field_name` helper and corresponding `FormBuilder#field_name`
method provide an Action View-compliant way of overriding a form field
element's `[name]` attribute (similar to `field_id` and
`FormBuilder#field_id` introduced in rails/rails#40127[][]).

```ruby
text_field_tag :post, :title, name: field_name(:post, :title, :subtitle)
  # => <input type="text" name="post[title][subtitle]">

text_field_tag :post, :tag, name: field_name(:post, :tag, multiple: true)
  # => <input type="text" name="post[tag][]">

form_for @post do |f|
  f.field_tag :tag, name: f.field_name(:tag, multiple: true)
  # => <input type="text" name="post[tag][]">
end
```

[rails/rails#40127]: https://github.com/rails/rails/pull/40127

actionview/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Introduce the `field_name` view helper, along with the
+    `FormBuilder#field_name` counterpart:
+
+    ```ruby
+    form_for @post do |f|
+      f.field_tag :tag, name: f.field_name(:tag, multiple: true)
+      # => <input type="text" name="post[tag][]">
+    end
+    ```
+
+    *Sean Doyle*
+
 *   Add `:day_format` option to `date_select`
 
         date_select("article", "written_on", day_format: ->(day) { day.ordinalize })

actionview/lib/action_view/helpers/form_helper.rb

@@ -1746,6 +1746,28 @@ module ActionView
         @template.field_id(@object_name, method, *suffixes, index: index)
       end
 
+      # Generate an HTML <tt>name</tt> attribute value for the given name and
+      # field combination
+      #
+      # Return the value generated by the <tt>FormBuilder</tt> for the given
+      # attribute name.
+      #
+      #   <%= form_for @post do |f| %>
+      #     <%= f.text_field :title, name: f.field_name(:title, :subtitle) %>
+      #     <%# => <input type="text" name="post[title][subtitle]">
+      #   <% end %>
+      #
+      #   <%= form_for @post do |f| %>
+      #     <%= f.field_tag :tag, name: f.field_name(:tag, multiple: true) %>
+      #     <%# => <input type="text" name="post[tag][]">
+      #   <% end %>
+      #
+      def field_name(method, *methods, multiple: false, index: @index)
+        object_name = @options.fetch(:as) { @object_name }
+
+        @template.field_name(object_name, method, *methods, index: index, multiple: multiple)
+      end
+
       ##
       # :method: text_field
       #

actionview/lib/action_view/helpers/form_tag_helper.rb

@@ -114,6 +114,32 @@ module ActionView
         end
       end
 
+      # Generate an HTML <tt>name</tt> attribute value for the given name and
+      # field combination
+      #
+      # Return the value generated by the <tt>FormBuilder</tt> for the given
+      # attribute name.
+      #
+      #   <%= text_field_tag :post, :title, name: field_name(:post, :title, :subtitle) %>
+      #   <%# => <input type="text" name="post[title][subtitle]">
+      #
+      #   <%= text_field_tag :post, :tag, name: field_name(:post, :tag, multiple: true) %>
+      #   <%# => <input type="text" name="post[tag][]">
+      #
+      def field_name(object_name, method_name, *method_names, multiple: false, index: nil)
+        names = method_names.map! { |name| "[#{name}]" }.join
+
+        # a little duplication to construct fewer strings
+        case
+        when object_name.empty?
+          "#{method_name}#{names}#{multiple ? "[]" : ""}"
+        when index
+          "#{object_name}[#{index}][#{method_name}]#{names}#{multiple ? "[]" : ""}"
+        else
+          "#{object_name}[#{method_name}]#{names}#{multiple ? "[]" : ""}"
+        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

@@ -105,15 +105,7 @@ module ActionView
           end
 
           def tag_name(multiple = false, index = nil)
-            # a little duplication to construct fewer strings
-            case
-            when @object_name.empty?
-              "#{sanitized_method_name}#{multiple ? "[]" : ""}"
-            when index
-              "#{@object_name}[#{index}][#{sanitized_method_name}]#{multiple ? "[]" : ""}"
-            else
-              "#{@object_name}[#{sanitized_method_name}]#{multiple ? "[]" : ""}"
-            end
+            @template_object.field_name(@object_name, sanitized_method_name, multiple: multiple, index: index)
           end
 
           def tag_id(index = nil)

actionview/test/template/form_helper_test.rb

@@ -1659,6 +1659,126 @@ class FormHelperTest < ActionView::TestCase
     assert_dom_equal expected, output_buffer
   end
 
+  def test_form_for_field_name_with_blank_as
+    form_for(Post.new, as: "") do |form|
+      concat form.text_field(:title, name: form.field_name(:title))
+    end
+
+    expected = whole_form("/posts", "new_", "new_") do
+      %(<input id="title" name="title" type="text">)
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_field_name_with_blank_as_and_multiple
+    form_for(Post.new, as: "") do |form|
+      concat form.text_field(:title, name: form.field_name(:title, multiple: true))
+    end
+
+    expected = whole_form("/posts", "new_", "new_") do
+      %(<input id="title" name="title[]" type="text">)
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_field_name_without_method_names_or_multiple_or_index
+    form_for(Post.new) do |form|
+      concat form.text_field(:title, name: form.field_name(:title))
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      %(<input id="post_title" name="post[title]" type="text">)
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_field_name_without_method_names_and_multiple
+    form_for(Post.new) do |form|
+      concat form.text_field(:title, name: form.field_name(:title, multiple: true))
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      %(<input id="post_title" name="post[title][]" type="text">)
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_field_name_without_method_names_and_index
+    form_for(Post.new, index: 1) do |form|
+      concat form.text_field(:title, name: form.field_name(:title))
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      %(<input id="post_1_title" name="post[1][title]" type="text">)
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_field_name_without_method_names_and_index_and_multiple
+    form_for(Post.new, index: 1) do |form|
+      concat form.text_field(:title, name: form.field_name(:title, multiple: true))
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      %(<input id="post_1_title" name="post[1][title][]" type="text">)
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_field_name_with_method_names
+    form_for(Post.new) do |form|
+      concat form.text_field(:title, name: form.field_name(:title, :subtitle))
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      %(<input id="post_title" name="post[title][subtitle]" type="text">)
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_field_name_with_method_names_and_index
+    form_for(Post.new, index: 1) do |form|
+      concat form.text_field(:title, name: form.field_name(:title, :subtitle))
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      %(<input id="post_1_title" name="post[1][title][subtitle]" type="text">)
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_field_name_with_method_names_and_multiple
+    form_for(Post.new) do |form|
+      concat form.text_field(:title, name: form.field_name(:title, :subtitle, multiple: true))
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      %(<input id="post_title" name="post[title][subtitle][]" type="text">)
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_field_name_with_method_names_and_multiple_and_index
+    form_for(Post.new, index: 1) do |form|
+      concat form.text_field(:title, name: form.field_name(:title, :subtitle, multiple: true))
+    end
+
+    expected = whole_form("/posts", "new_post", "new_post") do
+      %(<input id="post_1_title" name="post[1][title][subtitle][]" type="text">)
+    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

@@ -211,6 +211,66 @@ class FormTagHelperTest < ActionView::TestCase
     assert_equal "post_author_name", value
   end
 
+  def test_field_name_without_object_name
+    value = field_name("", :title)
+
+    assert_equal "title", value
+  end
+
+  def test_field_name_without_object_name_and_multiple
+    value = field_name("", :title, multiple: true)
+
+    assert_equal "title[]", value
+  end
+
+  def test_field_name_without_method_names_or_multiple_or_index
+    value = field_name(:post, :title)
+
+    assert_equal "post[title]", value
+  end
+
+  def test_field_name_without_method_names_and_multiple
+    value = field_name(:post, :title, multiple: true)
+
+    assert_equal "post[title][]", value
+  end
+
+  def test_field_name_without_method_names_and_index
+    value = field_name(:post, :title, index: 1)
+
+    assert_equal "post[1][title]", value
+  end
+
+  def test_field_name_without_method_names_and_index_and_multiple
+    value = field_name(:post, :title, index: 1, multiple: true)
+
+    assert_equal "post[1][title][]", value
+  end
+
+  def test_field_name_with_method_names
+    value = field_name(:post, :title, :subtitle)
+
+    assert_equal "post[title][subtitle]", value
+  end
+
+  def test_field_name_with_method_names_and_index
+    value = field_name(:post, :title, :subtitle, index: 1)
+
+    assert_equal "post[1][title][subtitle]", value
+  end
+
+  def test_field_name_with_method_names_and_multiple
+    value = field_name(:post, :title, :subtitle, multiple: true)
+
+    assert_equal "post[title][subtitle][]", value
+  end
+
+  def test_field_name_with_method_names_and_multiple_and_index
+    value = field_name(:post, :title, :subtitle, index: 1, multiple: true)
+
+    assert_equal "post[1][title][subtitle][]", value
+  end
+
   def test_hidden_field_tag
     actual = hidden_field_tag "id", 3
     expected = %(<input id="id" name="id" type="hidden" value="3" autocomplete="off" />)