Given a comments.json (output from a the /documents.json for regulations.gov api), download all the attachments for the comments.

download.rb

require 'JSON'
require 'open-uri'

api_key = "REPLACE_WITH_DATA_GOV_API_KEY"

file = File.open('comments.json')

comments_json = JSON.load(file)["documents"]

comments_with_attachments = comments_json.select { |comment| comment["attachmentCount"] > 0 }

comments_with_attachments.each do |comment|
  comment_id = comment["documentId"]
  comment_metadata = JSON.load(open("http://api.data.gov:80/regulations/v3/document.json?api_key=#{api_key}&documentId=#{comment_id}"))
  attachments = comment_metadata["attachments"]
  attachments.each do |attachment|
    attachment_order_number = attachment["attachmentOrderNumber"]
    attachmentURL = attachment["fileFormats"].first
    file_ext = /contentType=([^&]+)/.match(attachmentURL)[1]
    open("#{comment_id}_#{attachment_order_number}.#{file_ext}", "wb") do |file|
      file << open("#{attachmentURL}&api_key=#{api_key}").read
    end
  end
end