如何通过元编程覆盖Ruby on Rails模块方法？

Question

As much I would like to I still don't know how to implement metaprogramming.

For views rendered from my Admin::Base controller I would like override the Rails compute_public_path located under ActionView::Helpers::AssetTagHelper, so that all my admin's layout files will be under public/admin.

There is probably a better way to this, but at this point I want to learn how to do it this way.

I put this in Admin::Base controller:

module ActionView::Helpers::AssetTagHelper
  def compute_public_path(source, dir, ext = nil, include_host = true)
    super(source, "admin/#{dir}", ext = nil, include_host = true)
  end
end

but it gives me:

super: no superclass method `compute_public_path' for #<ActionView::Base:0x1032240a8>

Which doesn't surprise me.

---

If I try this in my admin helper:

def compute_public_path_with_admin(source, dir, ext = nil, include_host = true)
  compute_public_path_without_admin(source, "admin/#{dir}", ext, include_host)
end

alias_method_chain :compute_public_path, :admin

I get "undefined method 'compute_public_path' for module 'AdminHelper'", which I am guessing happens because compute_public_path is a private method.

I found this works:

ActionView::Helpers::AssetTagHelper.class_eval do
  def compute_public_path_with_admin(source, dir, ext = nil, include_host = true)
    compute_public_path_without_admin(source, "admin/#{dir}", ext, include_host)
  end

  alias_method_chain :compute_public_path, :admin
end

as long as I set config.cache_classes to false , otherwise I get a stack level too deep error.

Thanks to Squeegy for pointing me in the right direction.

How can I make this work with disabling class caching?

Answer 1

def compute_public_path_with_admin(source, dir, ext = nil, include_host = true)
  compute_public_path_without_admin(source, "admin/#{dir}", ext, include_host)
end

alias_method_chain :compute_public_path, :admin

compute_public_path will now call your implementation, which then calls the original implementation.

See: Is alias_method_chain synonymous with alias_method?

Answer 2

Please don't do this.

The accepted answer will get what you wanted, but what you want might have dangerous security implications.

Why would you want your admin layouts under public ? There's a reason why Rails by default puts them under app/views/layouts : No one will be able to read their source there.

If you put them in public , they are basically readable for any visitor.

If you put them where they belong, you're not exposing your layout code. And on top of that, you can structure app/views/layouts any way you want without any metaprogramming , ie without having to hack framework internals.