# Gemfile
gem 'rails-assets-jquery', source: 'https://rails-assets.org'
gem 'rails-assets-datatables', source: 'https://rails-assets.org'
gem 'kaminari'

# index.html.erb
<%= content_tag :table, role: :datatable, data: { url: users_path(format: :json)} do %>
  <thead>...</thead>
  <tbody></tbody>
<% end %>

# users_controller.rb
  def index
    respond_to do |format|
      format.html
      format.json { render json: UsersDatatable.new(view_context) }
    end
  end

# app/datatables/application_datatable.rb
class ApplicationDatatable
  delegate :params, to: :@view
  delegate :link_to, to: :@view

  def initialize(view)
    @view = view
  end

  def as_json(options = {})
    {
      recordsTotal: count,
      recordsFiltered: total_entries,
      data: data
    }
  end


private

  def page
    params[:start].to_i / per_page + 1
  end

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

  def sort_column
    columns[params[:order]['0'][:column].to_i]
  end

  def sort_direction
    params[:order]['0'][:dir] == "desc" ? "desc" : "asc"
  end
end

# app/datatables/user_datatable.rb
class UsersDatatable < ApplicationDatatable
  delegate :edit_user_path, to: :@view

  private

  def data
    users.map do |user|
      [].tap do |column|
        column << user.first_name
        column << user.last_name
        column << user.email
        column << user.phone_number

        links = []
        links << link_to('Show', user)
        links << link_to('Edit', edit_user_path(user))
        links << link_to('Destroy', user, method: :delete, data: { confirm: 'Are you sure?' })
        column << links.join(' | ')
      end
    end
  end

  def count
    User.count
  end

  def total_entries
    users.total_count
    # will_paginate
    # users.total_entries
  end

  def users
    @users ||= fetch_users
  end

  def fetch_users
    search_string = []
    columns.each do |term|
      search_string << "#{term} like :search"
    end

    # will_paginate
    # users = User.page(page).per_page(per_page)
    users = User.order("#{sort_column} #{sort_direction}")
    users = users.page(page).per(per_page)
    users = users.where(search_string.join(' or '), search: "%#{params[:search][:value]}%")
  end

  def columns
    %w(first_name last_name email phone_number)
  end
end

# application.js
//= require jquery
//= require datatables

...

$(document).on('turbolinks:load', function(){
  $("table[role='datatable']").each(function(){
    $(this).DataTable({
      processing: true,
      serverSide: true,
      ajax: $(this).data('url')
    });
  });  
})

# application.css
 *= require datatables
 *
 * For bootstrap support
 * require datatables/dataTables.bootstrap
 *