gistfile1.rb

module A
  class Bar
  end

  class Foo
    def bar
      # this works fine
      Bar.new
    end
  end
end

A::Foo.new.bar



module B
end

class B::Bar
end

class B::Foo
  def bar
    # this does not work: uninitialized constant B::Foo::Bar
    Bar.new
  end
end

B::Foo.new.bar