filtering results based on multiple criteria

What's the paradigm for providing a set of filters for search results?

Here's my example: I have a list of projects, and projects can be filtered by manager, status (open, closed, in progress), start date, etc. The filter settings are stored in the database using the preferences plugin, which works very nicely. However, my code is ugly, and I can't detect when a filter has be unset, which makes me think there is a better way.

ProjectsController:

# the if statement always returns true even if show_closed_projects is unchecked def index   @projects = Projects.all   if defined? params[:show_closed_projects] && params[:show_closed_projects] == 'on'     current_user.write_preference(:show_closed_projects, true)     @projects = @projects.closed # this is a named scope   else     current_user.wrirte_preference(:show_closed_projects, false)   end end

views/projects/index.html.erb # this works -- if the preference is set, the checkbox is checked, otherwise, it's not checked. <div id="project-filters" style="display:none" class="options">   <form action="/company/6/projects">     <fieldset>       <legend>project filter options</legend>       <table class="vertical-columns">         <tr>           <th colspan=2>             <input type="checkbox"               name="show_closed_projects"               <%=h current_user.prefers_show_closed_projects ? "checked" : "" %>             >             Show closed projects?           </th>         </tr>       </table>     </fieldset>     <p class="button-container">       <%= submit_tag "save filters" %>       <%= link_to "cancel" %>     </p>   </form> </div>

...projects listed here...