Fix MissingTemplate on Rails >= 8.0 with prepended controller modules

[giant-saimiri]

Fix MissingTemplate on Rails >= 8.0 when a module is prepended to the controller

Symptom

On Rails 8.0+, render xlsx: "show" raises:

ActionView::MissingTemplate (Missing template /show with {locale: [:en], formats: [:xlsx], ...})

— note the bare /show: the template is looked up at the view-paths root with no prefixes, instead of under the controller's view directory. This happens for any controller that has a module prepended to it (memo_wise, instrumentation/APM gems, etc.). On Rails <= 7.2 the same code works fine.

Root cause

Two things combine:

1. Rails 8.0 reordered render option normalization. Through Rails 7.2, _normalize_render ran before custom renderers, so options[:template] was already defaulted to the action name and options[:prefixes] to the controller's _prefixes by the time this gem's :xlsx renderer block executed — the gem's own if options[:template].nil? defaulting was dead code. Rails 8.0 moved that normalization into render_to_body (_normalize_options became _process_render_template_options in ActionView::Rendering), which runs after the renderer. So on 8.0 the gem's defaulting branch is exercised for the first time.

2. The gem's prefix computation breaks on prepended modules.

   options[:prefixes] ||= self.class.ancestors
                              .take_while { |a| a.respond_to?(:controller_path) }
                              .map(&:controller_path)

   A prepended module is the first entry in ancestors and does not respond to controller_path, so take_while stops immediately and returns []. With empty prefixes, the bare template name is searched at the view-paths root → MissingTemplate.

Fix

Use the controller's _prefixes instead. It is built from the superclass chain (local_prefixes + superclass._prefixes, see AbstractController::ViewPaths), so it is immune to prepended modules — and it is exactly the value Rails itself injected before 8.0 and still uses for a bare render :action, so behavior is identical for vanilla controllers.

Test

Adds Examples::PrependedModuleController (prepends an empty module to mirror what memo_wise & co. do) with a request spec. Without the fix it fails on Rails 8.0/8.1 with the exact error above; it passes on <= 7.2 either way, since there the normalization-order made the branch dead code. Suite verified green locally on Rails 7.1, 7.2, 8.0 and 8.1.

🤖 Generated with Claude Code

[giant-saimiri]

Superseded by #202 — same change, moved to our organization's fork. Sorry for the noise!