Bug #2740: Extend const_missing to pass in the nesting
http://redmine.ruby-lang.org/issues/show/2740

Author: Yehuda Katz
Status: Open, Priority: Urgent
Category: core, Target version: 1.9.2
ruby -v: ruby 1.8.7 (2008-08-11 patchlevel 72) [universal-darwin10.0] 

At the moment, it is impossible for const_missing to differentiate between these cases:

    class Foo::Bar
      Baz
    end
 
    class Foo
      class Bar
        Baz
      end
    end

In Rails, we implement a class loading system that loads classes from the file system if they are not found. In the above case, Foo::Baz might be stored in app/models/foo/baz.rb. We would like to be able to use the same Ruby constant lookup logic when looking up classes from the file system.

    class Foo::Bar
      Baz
    end

Here, we should look in "app/models/foo/bar/baz.rb" and in "app/models/baz.rb" just as Ruby would search for Foo::Bar::Baz and then Object::Baz.

    class Foo
      class Bar
        Baz
      end
    end

Here, we should look in "app/models/foo/bar/baz.rb", then "app/models/foo/baz.rb", and finally "app/models/baz.rb" just as Ruby would search for Foo::Bar::Baz, then Foo::Baz, and then Object::Baz.

In order to achieve this, I propose that we extend the const_missing method to take an optional second parameter containing the nesting:

    class Foo
      class Bar
        def self.const_missing(id, nesting)
          id == :Baz
          nesting == [Foo::Bar] # first case
          nesting == [Foo::Bar, Foo] # second case
        end
      end
    end

This would allow people who wish to do their own custom constant loading (such as Rails) to do so in a way that is consistent with Ruby's own constant lookup. In order to avoid backward-compatibility issues, we can check the arity of the const_missing method, and only pass in the nesting if a second parameter was provided.


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