Unexpected controller prefix path de-duplication in view partial lookup paths about rails HOT 2 OPEN

I've reviewed the history of this code and it's not clear what the intended behaviour should be.

The original implementation of ActionView::PartialRenderer#merge_path_into_partial introduced in dc1b0fd would remove any tokens that were repeated at the same position, so example 2 (Courses::Quiz::Question and Learning::Quiz::Extra::QuestionsController) would work as described.

The logic changed in 2995134 to add a break as soon as the first overlapping module was detected, but this ignores any subsequent modules, and only requires that the module depth is the same. This is the behaviour that still exists today.

There is very little test coverage to explain what behaviour the authors intended to support, and the behaviour as currently implemented is arguably wrong and at best confusing for users.

The case we could like to see addressed is where a controller has a module prefix that is not present on the object path, example above is a bit contrived, but I suspect many developers will have controllers in Admin modules. The additional prefix means that controller/object de-duplication does not work because the relative offsets are different.

merge_prefix_into_object_path("admin/quiz/questions_controller", "quiz/questions/_question.html.erb")
# => `admin/quiz/questions/_question.html.erb`

An alternative implementation would look for overlap at the end of the controller path and the beginning of the object path, and remove any duplication it detects. Here's an example implementation:

prefixes = []
prefix_array = File.dirname(prefix).split("/")

prefix_array.each_with_index do |dir, index|
  break if object_path.start_with?(prefix_array[index..-1].join("/"))
  prefixes << dir
end

(prefixes << object_path).join("/")

I'd be happy to provide a PR if this behaviour is acceptable.

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Previous discussion from 2011: #1951

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