Rails example of how to connect to the Gmail API using Ruby and download a Zip File attachment and then unzip contents from the zip archive

emailed_zip_file_extractor.rb

class EmailedZipFileExtractor
  # This class downloads an email from gmail and extracts a file out of a .zip attachment

  require 'google/apis/gmail_v1'
  require 'googleauth'
  require 'zip'

  # These credentials come from creating an OAuth Web Application client ID
  # in the Google developer console
  #
  # refer: https://www.youtube.com/watch?v=hfWe1gPCnzc
  #
  # ie.
  # > visit http://console.developers.google.com
  # > API Manager
  # > Credentials
  # > Create Credentials (OAuth client ID)
  # > Application type: Web Application
  # > Authorised redirect URIs: https://developers.google.com/oauthplayground
  # * the resultant client ID / client secret goes in the following GOOGLE_KEY / GOOGLE_SECRET variables
  # > visit: https://developers.google.com/oauthplayground/
  # > Click the settings icon to show the OAuth 2.0 configuration
  # > Tick 'Use your own OAuth credentials'
  # > Enter the OAuth Client ID and OAuth Client secret that you have just created
  # > Check the entry for 'Google Play Developer API v2' in the scopes field and click 'Authorize APIs'
  # > Click 'Allow'
  # > Click 'Exchange authorization code for tokens'
  # * the resultant Refresh token and Access token go in the following REFRESH_TOKEN / ACCESS_TOKEN variables
  GOOGLE_KEY    = 'xxxcc.apps.googleusercontent.com'
  GOOGLE_SECRET = 'abcd'
  ACCESS_TOKEN  = 'abcd'
  REFRESH_TOKEN = 'abcd'

  TMP_DIR = "#{Rails.root.to_s}/tmp/list"
  APPLICATION_NAME = 'Rails App'
  EMAIL_ADDRESS = "[email protected]"
  GMAIL_QUERY = "from:[email protected] has:attachment subject:Email+Subject"
  Gmail = Google::Apis::GmailV1

  def initialize
    FileUtils.rm_r(TMP_DIR) if File.directory?(TMP_DIR)
    FileUtils.mkdir_p(TMP_DIR)
  end

  def fetch
    File.read(file_path)
  end

private

  def file_path
    "#{TMP_DIR}/file.txt".tap do |path|
      Zip::File.open(zip_file_path) do |zip_file|
        # Match the required file
        entry = zip_file.glob('full_list/_reliable_list.txt').first
        entry.extract(path)
      end
    end
  end

  def zip_file_path
    "#{TMP_DIR}/archive.zip".tap do |path|
      File.open(path, "wb"){|f| f.puts attachment.data }
    end
  end

  def attachment
    gmail.get_user_message_attachment(EMAIL_ADDRESS, message_id, attachment_id)
  end

  def attachment_id
    email.payload.parts.find{|p| p.mime_type == 'application/zip'}.body.attachment_id
  end

  def email
    gmail.get_user_message(EMAIL_ADDRESS, message_id)
  end

  def message_id
    # Get the latest email that matches the query
    @message_id ||= gmail.list_user_messages(EMAIL_ADDRESS, q: GMAIL_QUERY).messages.first.id
  end

  def gmail
    @gmail ||= Gmail::GmailService.new.tap do |publisher|
      publisher.authorization = client
      publisher.client_options.application_name = APPLICATION_NAME
    end
  end

  def client(scopes = [Google::Apis::GmailV1::AUTH_GMAIL_READONLY])
    Signet::OAuth2::Client.new(
      authorization_uri: 'https://accounts.google.com/o/oauth2/auth',
      token_credential_uri: 'https://accounts.google.com/o/oauth2/token',
      client_id: GOOGLE_KEY,
      client_secret: GOOGLE_SECRET,
      access_token: ACCESS_TOKEN,
      refresh_token: REFRESH_TOKEN,
      scope: Google::Apis::GmailV1::AUTH_GMAIL_READONLY,
    )
  end

end