csv_renderer.rb

require 'csv'

ActionController::Renderers.add :csv do |collection, options|
  self.content_type ||= Mime::CSV
  self.headers['Content-Disposition'] = "attachment; filename=#{options[:filename]}.csv" if options[:filename]
  self.response_body = collection.to_csv
end

module CsvRenderer
  def to_csv(options={})
    columns = column_names
    columns += options[:include] if options[:include]
    CSV.generate do |csv|
      csv << columns
      all.pluck(*columns).each do |row|
        csv << row
      end
    end
  end
end

ActiveRecord::Base.extend CsvRenderer

item.rb

class Item < ActiveRecord::Base
  def self.to_csv
    super(include: [:username, :first_name])
  end
end

items_controller.rb

class ItemsController < OpsController
  respond_to :json, :csv

  def index
    respond_with(Item.joins(:user), filename: "items")
  end
end