title: Spine.js Model: Only first .save() should create new resource, all following should update
body: <p>When I create and .save() a new Record, my Spine.js Model sends a POST Request to the server to add it to the database.
  No big deal...
  But when I issue another .save() to the same model instance I want Spine.js to send a PUT Request just updating it's data.</p>
  
  <p>How can I achieve this?</p>
  
  <p>This is how my model looks like:</p>
  
  <pre><code>class App.MeetingMinutes extends Spine.Model
    @configure 'MeetingMinutes', 'subject', 'some', 'more', 'attributes'
    @belongsTo 'archive', 'Archive'
    @extend Spine.Model.Ajax
  
    @url: '#' # only dynamically scoped from Entries controller
  
    @setArchive: ( _id ) -&gt;
      @url = '/archives/' + _id + '/meeting_minutes'
  </code></pre>
  
  <p>(Adding an id column to @configure doesn't work either)</p>
  
  <p>In my Rails controller (create action) I return the whole Object as JSON</p>
  
  <pre><code>format.json { render :json =&gt; @meeting_minutes, :status =&gt; :created }
  </code></pre>
  
  <p>And that's what I'm using it:</p>
  
  <pre><code>mm = new App.MeetingMinutes({subject: 'subject'})
  mm.save() // POST
  mm.subject = 'changed subject!'
  mm.save() // another POST
  
  mm.isNew() // true (obviously :/)
  </code></pre>
  
  <p>Any ideas?</p>