Rails remember form checkbox value

 <% User.tag_counts_on(:tags).each do |tag| %>
    <%= f.check_box(:tag_list, { :multiple => true }, tag, nil ) %>
    <li class="tag-name"><%= tag.name %></li>
 <% end %>

The code above doesn't remember the selected value when the validation fails on sign up. Also on the edit page it doesn't remember what my selected tags are. Any suggestions?