class UserController < ActiveRecord::Base
  # 〜略〜
  def index
    @search = User.search(params[:search])
    @users = @search.relation.latest
    # csv以外ならページング付
    @users = @users.page(params[:page]) unless request.format == Mime::CSV
    respond_to do |format|
      format.html # index.html.erb
      format.json { render json: @users }
      format.csv  { send_data User.csv(@users), type: 'text/csv; charset=shift_jis', filename: "hoge.csv" }
    end
  end
  # 〜略〜
end

class User
  # 〜略〜
  def self.csv(users)
    require 'csv'
    headers = %w(名前 住所 メモ)
    # force_quotesオプションで全fieldにクオート強制される
    csv_data = CSV.generate(headers: headers, write_headers: true, force_quotes: true) do |csv|
      enum = if users.is_a?(Array)
        # Arrayだったらそのまま
        users.each
      else
        # Relationとかだったらfind_in_batchesで分割して
        Enumerator.new do |yielder|
          users.find_in_batches do |records|
            records.each { |record| yielder.yield record }
          end
        end
      end
      enum.each do |user|
        csv << [
          user.name,
          user.address,
          user.note
        ]
      end
    end
    csv_data.encode(Encoding::SJIS) # sjisにへんかーん
  end
  # 〜略〜
end

class AddMaxAttemptsToDelayedJobs < ActiveRecord::Migration
  def change
    add_column :delayed_jobs, :max_attempts, :integer
  end
end

if defined?(Delayed::Backend::ActiveRecord::Job)
  module JobWithMaxAttemptsAttribute
    def self.included(base)
      base.class_eval do
        attr_accessible :max_attempts
        alias_method_chain :max_attempts, :attribute
      end
    end
    def max_attempts_with_attribute
      self[:max_attempts] || max_attempts_without_attribute
    end
  end
  Delayed::Backend::ActiveRecord::Job.send(:include, JobWithMaxAttemptsAttribute)
end

HogeMailer.delay(max_attempts: 1).fuga(record)