active_admin_extensions.rb

module ActiveAdmin
    
  module Reports    
    module DSL
      def enable_reports
        action_item only: :index do
          link_to("Download", {action: :report, params: params}, {method: :post, data: { confirm: "Are you sure you want to generate this report?"}})
        end
  
        collection_action :report, method: :post do
          report = Report.create({
            name: resource_class.to_s.pluralize.titleize,
            model_name: resource_class.to_s,
            admin_user: current_admin_user,
            params: params['q']
          })
          Report.delay.generate(report.id)    
          redirect_to(admin_report_path(report.id), {notice: "Report queued for processing!"})
        end
      end
    end
  end
    
  ActiveAdmin::ResourceDSL.send :include, ActiveAdmin::Reports::DSL
end
  

csv_builder.rb

# CSVBuilder stores CSV configuration
# Extracted from ActiveAdmin
#
# Usage example:
#
#   csv_builder = CSVBuilder.new
#   csv_builder.column :id
#   csv_builder.column("Name") { |resource| resource.full_name }
#
#   csv_builder = CSVBuilder.new :col_sep => ";"
#   csv_builder.column :id
#
#
class CSVBuilder

  # Return a default CSVBuilder for a resource
  # The CSVBuilder's columns would be Id followed by this
  # resource's content columns
  def self.default_for_resource(resource)
    new(resource: resource).tap do |csv_builder|
      csv_builder.column(:id)
      resource.content_columns.each do |content_column|
        csv_builder.column(content_column.name.to_sym)
      end
    end
  end
  
  def self.call_method_or_proc_on(receiver, *args)
    options = { :exec => true }.merge(args.extract_options!)

    symbol_or_proc = args.shift

    case symbol_or_proc
    when Symbol, String
      receiver.send(symbol_or_proc.to_sym, *args)
    when Proc
      if options[:exec]
        instance_exec(receiver, *args, &symbol_or_proc)
      else
        symbol_or_proc.call(receiver, *args)
      end
    end
  end

  attr_reader :columns, :options

  def initialize(options={}, &block)
    @resource = options.delete(:resource)
    @columns, @options = [], options
    instance_exec &block if block_given?
  end

  # Add a column
  def column(name, &block)
    @columns << Column.new(name, @resource, block)
  end

  class Column
    attr_reader :name, :data

    def initialize(name, resource = nil, block = nil)
      @name = name.is_a?(Symbol) && resource.present? ? resource.human_attribute_name(name) : name.to_s.humanize
      @data = block || name.to_sym
    end
  end
end

report.rb

class Report < ActiveRecord::Base
  belongs_to :admin_user
  
  # see https://github.com/thoughtbot/paperclip/wiki/Encryption-on-amazon-s3
  has_attached_file :report, s3_permissions: :private,
    s3_headers: { "x-amz-server-side-encryption" => "AES256" },
    path: "private/reports/:filename"
  
  # fog_host contains the protocol, but expiring url appends http to it so we have to remove that
  def download_url
    url = report.try(:expiring_url, Time.now + 60.seconds)
    url.gsub!('http://https://', 'https://') if url
  end
  
  def generate
    start!
    path = build_csv
    finish!
    self.report = File.open(path)
    uploaded!
  end
  
  def start!
    self.started_at = Time.now
    save!
  end
  
  def finish!
    self.finished_at = Time.now
    save!
  end
  
  def uploaded!
    self.uploaded_at = Time.now
    save!
  end
  
  def uploaded?
    uploaded_at?
  end
  
  def status
    if uploaded_at?
      :available
    elsif finished_at?
      :finished
    elsif started_at?
      :started
    else
      :pending
    end
  end
  
  private
  
    def build_csv
      model = model_name.classify.constantize
      path = "#{Rails.root.to_s}/tmp/#{filename}"
    
      csv_builder = model.csv_builder
      columns = csv_builder.columns

      record_count = 0
      CSV.open(path, "wb") do |csv|
        csv << columns.map(&:name)
        model.report_scope.search(params).result.find_each do |resource|
          csv << columns.map do |column|
            CSVBuilder.call_method_or_proc_on resource, column.data
          end
          record_count += 1
        end      
      end
      
      self.record_count = record_count
      path
    end
  
    def filename
      [model_name.try(:downcase), created_at.try(:to_i), id].join("_") + ".csv"
    end
  
  class << self
    def generate(report_id)
      find(report_id).generate
    end
  end
  
end

reportable.rb

# a mixin for your models to add default report behavior
module Reportable
  extend ActiveSupport::Concern
  
  def to_csv
    self.class.csv_builder.columns.map do |column|
      CSVBuilder.call_method_or_proc_on self, column.data
    end
  end
  
  module ClassMethods
    
    def csv_builder 
      CSVBuilder.default_for_resource(self)
    end
    
    def csv_columns
      csv_builder.columns.map(&:name)
    end
    
    def report_scope
      all
    end

  end
end

user.rb

# how to define the csv export inside of the model
class User < ActiveRecord::Base
  include Reportable
  
  # override some of the reportable methods
  
  class << self
    
    def report_scope
      # defaults to 'all' but you could do a custom scope like this...
      joins(:addresses)
    end
    
    def csv_builder
      csv = CSVBuilder.new({ resource: self }) do
        column :id
        column :user_id
        column :first_name
        column :last_name
        column("Address")       { |user| user.primary_address.try(:to_str) }
        column :created_at
      end
    end
  end
  
end

users.rb

#admin/users.rb
ActiveAdmin.register User do
  enable_reports
end