title: Rails has_many :through saving additional fields
body: <p>I am trying to find a graceful way of saving an additional field called description on the Appointment model (below). My models are setup like this:</p>
  
  <pre><code>class Physician &lt; ActiveRecord::Base
    has_many :appointments
    has_many :patients, through: :appointments
  end
  
  class Appointment &lt; ActiveRecord::Base
    belongs_to :physician
    belongs_to :patient
  end
  
  class Patients &lt; ActiveRecord::Base
    has_many :appointments
    has_many :physicians, through: :appointments
    attr_accessible :name
  end
  </code></pre>
  
  <p>In my view I have checkboxes setup to save the data for the join table but I want to slide in an additional "description" field to be saved with the join. Below is what is in my view:</p>
  
  <pre><code>&lt;div class="field"&gt;
    &lt;fieldset&gt;
    &lt;legend&gt;Patients&lt;/legend&gt;
    &lt;% @patients.each_slice(2) do |slice| %&gt;
      &lt;div class='row'&gt;
        &lt;% slice.each do |patient| %&gt;
          &lt;div class='span3'&gt;
            &lt;%= label_tag "physician_patient_ids_#{patient.id}" do %&gt;
              &lt;%= check_box_tag 'physician[patient_ids][]', patient.id,
                                @physician.patients.include?(patient),
                                { id: "physician_patient_ids_#{patient.id}" } %&gt;
              &lt;%= patient.name %&gt;
            &lt;% end %&gt;
            &lt;!-- need to add in description here somehow --&gt;
          &lt;/div&gt;
        &lt;% end %&gt;
      &lt;/div&gt;
    &lt;% end %&gt;
    &lt;/fieldset&gt;
  &lt;/div&gt;
  </code></pre>