title: Adding and removing from a has_many :through relation
body: <p>From the Rails associations guide, they demonstrate a many-to-many relationship using has_many :through like so:</p>
  
  <pre><code>class Physician &lt; ActiveRecord::Base
    has_many :appointments
    has_many :patients, :through =&gt; :appointments
  end
  
  class Appointment &lt; ActiveRecord::Base
    belongs_to :physician
    belongs_to :patient
  end
  
  class Patient &lt; ActiveRecord::Base
    has_many :appointments
    has_many :physicians, :through =&gt; :appointments
  end
  </code></pre>
  
  <p>How would I create and remove appointments?</p>
  
  <p>If I've got a <code>@physician</code>, do I write something like the following for creating an appointment?</p>
  
  <pre><code>@patient = @physician.patients.new params[:patient]
  @physician.patients &lt;&lt; @patient
  @patient.save # Is this line needed?
  </code></pre>
  
  <p>What about code for deleting or destroying? Also, if a Patient no longer existed in the Appointment table will it get destroyed?</p>