## my controller

class AttendanceController < ApplicationController

  def index
    meeting_id = params[:id]
    unless meeting_id
      flash[:message] = "you can take attendance without specifying a meeting"
      redirect_to admin_index
    end
    @my_meeting = Meeting.find(meeting_id)
    @time_slot = @my_meeting.time_slot
    @boot_camp = @time_slot.boot_camp
    @potential_users = User.find :all, :select => 'distinct users.*',
      :joins => {:orders => {:registrations => :time_slot}},
      :conditions => ['time_slots.id = ?',@my_meeting.time_slot.id]
    # ^^^^^^^^ why does that work?
    @meeting_users = MeetingUser.find(:all,
      :conditions => ['meeting_id = ?',@my_meeting.id])
  end

  def take_attendance
    params[:user_ids] ||= []
    users = params[:user_ids]
    unless (users.empty? || params[:meeting_id].empty? || !request.post?)
      meeting_id = params[:meeting_id].to_i
      @existing_attendees = \
        Meeting.find(meeting_id).meeting_users.map{|mu| mu.user}
      users.each do |user|
        if @existing_attendees.detect{|ea| ea.id = user.id} ||
          # they already are attending and we say they are
        elsif  # the user was checked, but now is not

        else # the user was never checked, create
          mu = MeetingUser.create(:user_id => user.id,
            :meeting_id => meeting_id)

        end
      end
    end

  end
end

## my view

<div class="floatLeft width25">
  <!-- Left thumbnail column -->

  <!-- Left thumbnails end -->
</div>
<!-- Left thumbnails end -->
<!-- Right column -->
<div class="floatRight width75">
  <h1><%= attendance_event(@boot_camp,@my_meeting,@time_slot) %></h1>
  <% if @potential_users.empty? %>
    <p>There are no potential users for
    the time slot from <%= @time_slot.start_to_finish %></p>
  <% else %>
          <% form_for :attendees,
              :url => {:action => 'take_attendance'} do |f| %>
            <%= hidden_field_tag 'meeting_id', @my_meeting.id %>
            <% @potential_users.each do |usr| %>
                    <%= check_box_tag "user_ids[]",
                              usr.id,
                              @meeting_users.include?(usr) %>
                    <%= usr.full_name %><br />
            <% end %>
      <p>
        <%= f.submit "Update" %>
      </p>
    <% end %>
<% end %>

<%= link_to "Return to meetings for #{@boot_camp.title}",
        boot_camp_time_slot_url(@boot_camp.id,@my_meeting.time_slot) %>
</div>