Action View: Fallback to existing partial when possible

[seanpdoyle]

Closes #50844

Motivation / Background

A controller declared in the top-level module can render a top-level Active Model instance whose partial is declared in the root view directory (like articles/_article.html.erb).

A controller scoped within a module can render an Active Model instance whose partial is similarly scoped within view directory (like scoped/articles/_article.html.erb).

A controller scoped within a module cannot render an Active Model instance whose partial is declared in the root view directory (like articles/_article.html.erb), despite the absence of a similarly scoped partial.

This is intended behavior that's powered by
config.action_view.prefix_partial_path_with_controller_namespace = true (true by default).

This change was introduced in March of 2012 as part of #5625.

Detail

As a consumer of Action View, my intuition is that the lookup would fallback, in the same way that a controller that inherits from ApplicationController could define its own view, then rely on fallback to render an app/views/application partial.

This commit modifies the behavior to gracefully fall back to the root-level view partial.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[rafaelfranca]

I think the behavior makes sense but we should not be manipularing the partial name to get the right object. This should probably be implemented in the lookup object not in render

[seanpdoyle]

While this passes the suite, this change feels too far removed from the original problem. The resolved directory is coincidentally the path name pluralized.

I believe resolving the issue will require changes to how the ObjectRenderer constructs its set of prefixes to prepend to the ViewPath (depending on the config.action_view.prefix_partial_path_with_controller_namespace configuration value).

[ansonhoyt]

I like where you're going with this. 👍 You may want consider another case in your testing. Your current solution adds a search combination that strips all namespaces, which doesn't seem to cover when a controller-namespaced view would like to render a model from another namespace...

For example, if Scoped::Article had an ActiveStorage file attribute:

class Scoped::Article < ApplicationRecord
  has_one_attached :file
end

The file attribute is is namespaced under ActiveStorage, with to_partial_path of "active_storage/attachments/attachment".

If we try to render with two partials:

<%# app/views/scoped/articles/_article.html.erb %>
Rendered controller-namespaced article
<%= render article.file %>

<%# app/views/active_storage/attachments/_attachment.html.erb %>
Rendered namespaced attachment

Your current idea removes both "scoped" and "active_storage" namespaces, so the attachment's partial wouldn't be found with:

- search_combinations(*["scoped/active_storage/attachments", "attachments"])
# => ActionView::MissingTemplate - Missing partial scoped/active_storage/attachments/_attachment

Would it make sense to add search paths as each namespace is removed?:

+ search_combinations(*["scoped/active_storage/attachments", "active_storage/attachments", "attachments"])

Then folks aren't forced to pick all-or-nothing namespacing, but can use any combination that suits their needs. Perhaps they'd have an "override" partial in one namespace, but use a general one elsewhere.

Thanks for your work on this!