title: Rails join tables
body: <p>I have the following models</p>
  
  <pre><code>User
  Meeting
  Participant
  
  Particpant &lt; User
  Meeting belongs_to :user
  Meeting has_many :participants
  </code></pre>
  
  <p>I have the following join table </p>
  
  <pre><code>create_table 'meetings_participants', :id =&gt; false do |t|
    t.column :meeting_id, :integer
    t.column :participant_id   , :integer
  end
  </code></pre>
  
  <p>I want to add t.column :confirmed_at , :datetime so that I know when and if a user has confirmed that they are attending a meeting but I dont know the best way to do it.</p>
  
  <p>How can I set confirmed_at?  Since this is </p>