Recently there was a bug reported to rails which results in an infinite loop when passing a form builder object into a partial.

<%= form_for [:admin, @author], html: { class: 'form form-horizontal', multipart: true, style: 'margin-bottom: 0' } do |f| %>
  <%= render partial: 'my_partial', locals: { f: f } %>
<% end %>

This would result in:

Completed 500  in 1701ms (ActiveRecord: 1314.9ms | Allocations: 163362)

SystemStackError (stack level too deep):

app/views/admin/authors/_form.html.erb:357

The bug report, and following comments which acknowledged the bug were helpful, but there was no simple way to reproduce it without building an app to test.

I was able to come up with an executable bug report, which I wanted to share because I think it's not well documented how to test Action View with paths in a single file. This is very useful for investigating bugs and various behaviors in the framework.

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "rails"
end

require "minitest/autorun"
require "action_view"

class BugTest < ActionView::TestCase
  def setup
    @controller.append_view_path "."
  end

  def test_stuff
    partial = <<~ERB
      <%= puts f %>
    ERB

    File.write("_partial.html.erb", partial)

    render inline: <<~ERB
      <%= form_with model: false, url: false do |f| %>
        <%= render(partial: 'partial', locals: { f: f }) %>
      <% end %>
    ERB
  end
end

However, the script could not reproduce the infinite loop bug.

As one person pointed out, the issue was actually not Rails, but a third-party gem meta_request and removing it would resolve the issue.

This is still a really helpful script, and I want to re-use it so I'm sharing it here.