title: counting queryset returns 'list' object has no attribute 'id'
body: <p>I have a model with a manytomany column. I want to limit the number of groups a user can submit. </p>
  
  <p>Here is what I'm doing</p>
  
  <pre><code>        group_name = [g.id for g in form.cleaned_data['groups']]
          gr=group_name.id.count()
          if gr&gt;3:
              raise forms.ValidationError("you can only add 3 groups")
          return group_name
  </code></pre>
  
  <p>the error is:</p>
  
  <pre><code> Exception Value: 'list' object has no attribute 'id'
  </code></pre>
  
  <p>I can't seem to find what I'm doing wrong. Thanks.</p>