CustomMailExporter

class CustomMailExporter
  require 'fileutils'

  attr_accessor :service, :target_users, :target_start_date, :target_end_date,
                :path, :errors, :filename_counter, :emails_found

  # Dates should be in Date or DateTime format.  Users should be an array.
  def initialize(service, target_users, target_start_date, target_end_date)
    @service = service
    @target_users = target_users.map(&:downcase)
    @target_start_date = target_start_date
    @target_end_date = target_end_date
    @path = "/mnt/#{service.id}/"
    @errors = []
    @filename_counter = 0
    @emails_found = 0
  end

  def process
    FileUtils.mkdir_p(path) unless File.directory?(path)

    service.metadatum_class.find_each(service.id) do |datum|
      if target_user?(datum.from) && date_in_range?(datum.date)
        write_contents_to_file(datum)
        @emails_found += 1
      end
    end
    return true if errors.count == 0
  end

  def convert_to_email_address(from)
    from.split("<").last.split(">").first
  end

  def target_user?(from)
    target_users.include?(convert_to_email_address(from).downcase)
  end

  def date_in_range?(date)
    date > target_start_date && date < target_end_date
  end

  def write_contents_to_file(datum)
    begin
      path_and_name = path
      if datum.respond_to?(:content_filename) && datum.content_filename.present?
        path_and_name += datum.content_filename.gsub(/[.<>:"\/\\|\?\*']/, "")
        #use gsub for problem characters in subjects
      else
        path_and_name += filename_counter.to_s
        filename_counter += 1
      end

      path_and_name += ".eml" unless path_and_name.include? ".eml"
      File.open(path_and_name, 'wb') do |f|
        datum.content { |chunk| f << chunk }
      end

    rescue => e
      errors << [datum.key, e]
    end
  end
end

Also regarding date_in_range?, the comparison you're making, date > target_start_date && date < target_end_date, suggests that target_start_date and target_end_date are non-inclusive. Is this desirable?

Minor FYI, but S3Datum has a method for writing content to a file, see Concerns::S3Datum#write_content_to_file for details.

Regarding lines 46-52, I dont think that you need to worry about Datums missing or not responding to content_filename. The content_filename method is required by the S3Datum interface for files to be stored in S3. Without it, we wouldn't be able to store or fetch content for them anyway. Also, with regard to

path_and_name += datum.content_filename.gsub(/[.<>:"\/\\|\?\*']/, "")
#use gsub for problem characters in subjects

it seems like the GoogleMailDatum and GoogleMailRestModels::CannonicalDatum classes both use the message_id to generate the filename, not the subject, so the gsub call may not be completely necessary, though I guess it's possible that I'm missing something.

Overall, I think it's good. Nice job!