_header.html.erb

<nav class="navbar navbar-default navbar-fixed-top navbar-inverse" role="navigation">
  <div class="container">
    <div class="row">
      <div class="col-md-4">
        <a class="navbar-brand" href="/"><%= image_tag "vinylhivelogo.png", :class => "logo" %>Vinyl<span class="logo_highlight">HIVE</span></a>
      </div>

      <div class="col-md-8">
        <div class="login_controls navbar-right">
          <% if current_user %>
            Signed in as <%= current_user.name %> / <%= link_to "Logout", '/logout' %>
          <% else %>
            <%= link_to 'Login', '/sessions/new', "data-toggle" => "modal", "data-target" => "login_modal", :remote => true %> / <%= link_to 'Signup', '/signup' %>
          <% end %>
        </div>
      </div>
  </div>
</nav>

<div id="login_modal" class="modal hide fade" tabindex="-1"></div>

_login_modal.html.erb

<div class='modal-header'>
  <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
  <h3 class="modal-title">VinylHIVE</h3>
</div>
<div class='modal-body'>
  <div class="modal-body-content">
    <%= form_for :sessions, :url => { :controller => "sessions", :action => "create" }, :method => :post, :remote => true do |f| %>
      Email: <%= text_field_tag :email %>
      Password: <%= password_field_tag :password %>
      <%= f.submit "Submit" %>
    <% end %>
  </div>
  <div class="ajax-loader"></div>
</div>
<div class='modal-footer'>
  <button type="button" data-dismiss="modal" class="btn">Close</button>
</div>

new.js.erb

$("#login_modal").html("<%= escape_javascript(render 'layouts/login_modal') %>");
$('#login_modal').modal('show');

sessions_controller.rb

class SessionsController < ApplicationController

  def new
  end

  def create
    user = User.find_by_email(params[:email])
    if user && user.authenticate(params[:password])
      session[:user_id] = user.id
      redirect_to '/'
    else
      respond_to do |format|
        format.js { render 'new' }
      end
    end
  end

  def destroy
    session[:user_id] = nil
    redirect_to '/login'
  end

end