application_controller.rb

# app/controllers/application_controller.rb

class ApplicationController < ActionController::Base

  protected
  # for datatables
  def page
    params[:iDisplayStart].to_i/per_page + 1
  end

  def per_page
    params[:iDisplayLength].to_i > 0 ? params[:iDisplayLength].to_i : 10
  end

  def sort_column
    columns ||= []
    columns[params[:iSortCol_0].to_i]
  end

  def sort_direction
    params[:sSortDir_0] == "desc" ? "desc" : "asc"
  end
end

csv_renderer.rb

# config/csv_renderer.rb

ActionController::Renderers.add :csv do |obj, options|
  filename = options[:filename] || 'data'
  str = obj.respond_to?(:to_csv) ? obj.to_csv : obj.to_s
  send_data str, :type => Mime::CSV,
    :disposition => "attachment; filename=#{filename}.csv"
end

index.json.jbuilder

# app/views/posts/index.json.jbuilder

json.sEcho params[:sEcho].to_i
json.iTotalRecords Post.count
json.iTotalDisplayRecords @posts.total_count
posts = Array.new

@posts.each do |post|
  posts << [
    link_to(post.id, edit_post_path(post)),
    post.title,
    post.subtitle,
    post.descriptions,
    post.author
    (post.publish_date.strftime("%d-%m-%Y") unless post.publish_date.blank?),
  ]
end

json.aaData posts

post.rb

# app/models/post.rb

class Post < ActiveRecord::Base

  # other codes ...

  def self.to_csv(posts, options={})
    if posts.first.class.superclass == ActiveRecord::Base
      which_columns = export_columns
      CSV.generate do |csv|
        csv << which_columns
        posts.each do |row|
          csv << which_columns.collect{ |attribute| row[attribute] }
        end
      end
    else
      CSV.generate_line(posts, Hash.new)
    end
  end

  def self.export_columns
    columns ||= %w[id title subtitle author descriptions]
  end

end

posts_controller.rb

# app/controllers/posts_controller.rb

class PostsController < ApplicationController

  def index
    @posts = Post.order("#{sort_column} #{sort_direction}").page(page).per(per_page)
    if params[:sSearch].present?
      sql = "
        title like :search
        or subtitle like :search
        or descriptions like :search
        or author like :search"
      @posts = @posts.where(sql, search: "%#{params[:sSearch]}%")
    end

    respond_to do |format|
      format.json
      format.html
      format.csv { render :csv => Post.to_csv(@posts, columns:params[:aoColumns])}
    end
  end

protected
  def sort_column
    @columns ||= Post.export_columns
    # or 
    # @columns || = [id title subtitle descriptions author]
    @columns[params[:iSortCol_0].to_i]
  end
end