index.html.erb

<h1>Listing people</h1>

<table>
  <tr>
    <th><%= sorted_people_url("First name", "first_name") -%></th>
    <th><%= sorted_people_url("Last name", "last_name") -%></th>
    <th><%= sorted_people_url("Personal Email", "email") -%></th>
    <th><%= sorted_people_url("Age", "age") -%></th>
  </tr>

<% @people.each do |person| %>
  <tr>
    <td><%=h person.first_name %></td>
    <td><%=h person.last_name %></td>
    <td><%=h person.email %></td>
    <td><%=h person.age %></td>
    <td><%= link_to 'Show', person %></td>
    <td><%= link_to 'Edit', edit_person_path(person) %></td>
    <td><%= link_to 'Destroy', person, :confirm => 'Are you sure?', :method => :delete %></td>
  </tr>
<% end %>
</table>

<br />

<%= link_to 'New person', new_person_path %>

people_controller.rb

class PeopleController < ApplicationController
  # GET /people
  # GET /people.xml
  def index
    column, order = parse_order(params[:order])
    @people = Person.order_by(column, order)

    respond_to do |format|
      format.html # index.html.erb
      format.xml  { render :xml => @people }
    end
  end
  #... more actions here

  protected
  def parse_order(column_order_str)
    # if no column_order param, then return a default ordering
    # right now im just default ordering it by id desc, you can change
    # this to whatever
    return :id, :desc if column_order_str.nil?
    split = column_order_str.split("_")
    column = split[0..-2].join("_")
    order = split[-1]
    return column, order
  end
end

people_helper.rb

module PeopleHelper

  # column name is the name of html header column ("First Name", "Last Name", "Last Modified"
  # field is the field on the person model (database column: first_name, last_name, updated_at)
  # returns a link to #index with an order param specifying the order, if params[:order] exist and 
  # matches the column, the order is the opposite. 
  # ie params[:order] = "age_asc" then 
  # sorted_people_url("age", :age) # <a href="http://localhost:3000/people?order=age_desc">Age</a>
  #  
  def sorted_people_url(column_name, field)
    existing_field_order = params[:order].nil? ? nil : params[:order].downcase
    order = if existing_field_order &&
                existing_field_order.starts_with?(field.downcase) &&
                existing_field_order.ends_with?("desc")
              :asc
            else
              :desc
            end
    link_to(column_name, people_url(:order => "#{field}_#{order}"))
  end
 
end

person.rb

class Person < ActiveRecord::Base

  # column is the column to sort by
  # order is ASC or desc
  #
  named_scope(:order_by, lambda{|column, order| {:order => "#{table_name}.#{column} #{order}" }})
end