Fix the submitter’s formmethod not being respected when using Rails 7 #button form helper

app/javascript/turbo/fetch_requests.js

@@ -35,7 +35,16 @@ function determineFetchMethod(submitter, body, form) {
 
 function determineFormMethod(submitter) {
   if (submitter instanceof HTMLButtonElement || submitter instanceof HTMLInputElement) {
-    if (submitter.hasAttribute("formmethod")) {
+    // Rails 7 ActionView::Helpers::FormBuilder#button method has an override
+    // for formmethod if the button does not have name or value attributes
+    // set, which is the default. This means that if you use <%= f.button
+    // formmethod: :delete %>, it will generate a <button name="_method"
+    // value="delete" formmethod="post">. Therefore, if the submitter's name
+    // is already _method, it's value attribute already contains the desired
+    // method.
+    if (submitter.name === '_method') {
+      return submitter.value
+    } else if (submitter.hasAttribute("formmethod")) {
       return submitter.formMethod
     } else {
       return null

test/dummy/app/controllers/articles_controller.rb

@@ -33,6 +33,11 @@ def destroy
     redirect_to articles_url
   end
 
+  def destroy_all
+    Article.destroy_all
+    redirect_to articles_url, status: :see_other
+  end
+
   private
 
   def article_params

test/dummy/app/views/articles/edit.html.erb

@@ -10,6 +10,10 @@
 
   <button>Submit as PATCH</button>
   <button formmethod="post" formaction="<%= articles_path %>">Submit as POST</button>
+
+  <%# In Rails 7, there's an override of formaction, so `form.button formaction: destroy_all_articles_path, formmethod: :delete` would generate the HTML below. In Rails 6, the override of `formaction` does not exist yet, so we generate the HTML manually.  %>
+  <button name="_method" type="submit" formaction="<%= destroy_all_articles_path %>" formmethod="post" value="delete">Delete all articles</button>
+
 <% end %>
 
 <%= link_to "Submit as DELETE", article_path(@article.id), data: { turbo_method: :delete } %>

test/dummy/config/routes.rb

@@ -1,5 +1,7 @@
 Rails.application.routes.draw do
-  resources :articles
+  resources :articles do
+    delete :destroy_all, on: :collection
+  end
   resources :messages do
     collection do
       get :section