rails クエリを使って集計しcsvをexport

formatをcsvにする（ブラウザからcsvを要求する）には
linkのurlの拡張子をcsvにする必要あり

フォーム

form.slim

.form
  = form_tag 'output_csv.csv', method: :post do
    .form-group
      .mt10
        = datetime_select :csv, :start_at
      .mt10
        = datetime_select :csv, :finish_at
      .mt20.center
        = submit_tag "Output CSV", class: 'btn btn-primary'

コントローラ

chat_csvs_controller.rb

def output_csv
  start_at = datetime_params(params,'start_at')
  finish_at = datetime_params(params,'finish_at')
  csv_name = "chatbot#{start_at.strftime("%m/%d/%y")}to#{finish_at.strftime("%m/%d/%y")}.csv"
  @chats = Chat.count_desc(start_at,finish_at)
  respond_to do |format|
    format.csv do
      send_data render_to_string, filename: csv_name, type: :csv
    end
  end
end

モデル

chat.rb

class Chat < ActiveRecord::Base
  scope :count_desc, -> start_at, finish_at {
    Chat.select(:message,:created_at).where(created_at: start_at..finish_at).order('count_message desc').group(:message).count(:message)
  }
end

ビュー

output_csv.csv.ruby

require 'csv'
CSV.generate do |csv|
  column_names = %w(message count)
  csv << column_names
  @chats.each do |chat|
    csv_column_values = [
      chat[0],
      chat[1]
    ]
    csv << csv_column_values
  end
end