def flatten_keys_to_joined_string(h, key_delim='::')
  flat = {}
  h.each_pair do |k, v|
    if v.respond_to?(:keys)
      flatten_keys_to_joined_string(v, key_delim).each_pair do |k2, v2|
        flat.merge!({[k.to_s, k2.to_s].join(key_delim) => v2})
      end
    else
      flat.merge!({ k.to_s => v })
    end
  end

  flat
end