rubyzip - private method `open' called for Zip::File:Class (Ruby on Rails 4)

Question

I've added a helper method to my Application controller:

def unzip (file, destination)
    Zip::File.open(file) do |zip_file|
        zip_file.each do |f|
            f_path = File.join(destination, f.name)
            FileUtils.mkdir_p(File.dirname(f_path))
            f.extract(f_path) 
        end
    end
end

and I get the error:

private method `open' called for Zip::File:Class

When I run this in the Interactive Ruby shell, it works fine. How do I implement this on Rails 4?