LOGIN CONTROLLER
----------------

class LoginController < ApplicationController
  def index
    if @session[:valid]
      redirect_to :controller => "Admin"
    end
  end

  def validate_login(username, password)
    return true
  end

  def authenticate(username=params[:user][:username], password=params[:user][:password])
    if @session[:valid]
      redirect_to :controller => "Admin"
      return false
    end

    if @user = User.authenticate(username, password)
      @session[:valid] = true
      @session[:user_id] = @user.id
      @session[:username] = @user.username
      if @session[:return_to]
        redirect_to_path($session[:return_to])
        @session[:return_to] = nil
      else
        redirect_to :controller => "Admin"
      end
    else
      render :action => "index"
    end
  end


  def logout
    reset_session
  end
end



LOGIN MODEL
-----------

require 'digest/sha1'

class User < ActiveRecord::Base
  validates_presence_of :username, :password
  def self.authenticate(username, password)
    if password
      sha1password = Digest::SHA1.hexdigest(password)
    else
      sha1password = ''
    end
    return find(:first, :conditions => [ "username = ? AND password = ?", username, sha1password ])
  end
end



LOGIN VIEW
----------

<h1>Log In</h1>

<% if flash[:alert] %>
<div id="alert"><%= flash[:alert] %></div>
<% end %>

<%= start_form_tag :action => 'authenticate' %>
  <%= error_messages_for :user %>

  <!--[form:user]-->
    <p>
    <b>Username</b><br />
<% if params[:user] %>
    <%= text_field "user", "username", :size => '20', :value => params[:user][:username] %>
<% else %>
    <%= text_field "user", "username", :size => '20' %>
<% end %>
  </p>
  <p>
    <b>Password</b><br />
    <%= password_field "user", "password", :size => "20" %>
  </p>
  <!--[eoform:login]-->
  <%= submit_tag "Login" %>
<% end_form_tag %>