_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>

header.html.erb

<%= link_to 'Login', '/sessions/new', "data-toggle" => "modal", "data-target" => "#login_modal", :remote => true %> 
<div id="login_modal" class="modal fade" tabindex="-1" role="dialog" aria-hidden="true"></div>

new.js.erb

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