title: Cannot assign must be instance on form save
body: <p>I'm creating a site where users can publish their content, each content redirect to a private user to user chat depending on the content id.</p>
  
  <p>I'm doing it like this so I can retrieve the content infos on the private chat.</p>
  
  <p>The problem is that when a user submit the form to post a <code>comment</code> I get this error :</p>
  
  <blockquote>
    <p>Cannot assign "&lt; User: my_user >": "userComment.seller" must be a "Profile" instance.</p>
  </blockquote>
  
  <p>On my view, I'm saving the connected user as a <code>buyer</code> and the user who will receive the comment as <code>seller</code>.</p>
  
  <p>How can I fix this error ? What am I doing wrong on the code below ?</p>
  
  <p>here is <strong>models.py</strong> :</p>
  
  <pre><code>class Profile(models.Model):
      name = models.CharField(max_length=120)
      user = models.OneToOneField(User, null=True, blank=True)
  
  class Gig(models.Model):
      profile = models.ForeignKey(Profile, null=True)
      title = models.CharField(max_length=100, unique=True)
  
  class userComment(models.Model):
      buyer = models.ForeignKey(User, related_name="buyer", null=True)
      seller = models.ForeignKey(Profile, related_name="seller", null=True)
      comment = models.TextField(max_length=255, null=True)
  </code></pre>
  
  <p><strong>views.py</strong></p>
  
  <pre><code>def addComment(request, gig_id):
      gig = Gig.objects.get(id=gig_id)
      username = gig.profile.user
  
      if request.method == 'POST':
          form = userCommentForm(request.POST, request.FILES)
          if form.is_valid():
              form.save(buyer=request.user)
              return HttpResponseRedirect('/')
  
      else:
          form = userCommentForm()
          form.fields['seller'].initial = username
  </code></pre>
  
  <p><strong>forms.py</strong></p>
  
  <pre><code>class userCommentForm(forms.Form):
      seller = CommaSeparatedUserField(label=_(u"seller"))
      comment = forms.CharField(label=_(u"comment"), widget=forms.Textarea())
  
      def save(self, buyer):
          sellers = self.cleaned_data['seller']
          comment = self.cleaned_data['comment']
          message_list = []
          for seller in sellers:
              msg = userComment(buyer=buyer, seller=seller, comment=comment,)
  
              if seller == buyer:
                  print('-----------------------------------------------')
                  print('same')
  
              if None is not None:
                  msg.parent_msg = None
                  None.save()
              msg.save()
              message_list.append(msg)
  
          return message_list
  </code></pre>
  
  <hr>
  
  <p><strong>Traceback :</strong></p>
  
  <pre><code>File "C:\Python34\projects\tryTen\private_comment\views.py" in addComment
    31.             form.save(buyer=request.user)
  
  File "C:\Python34\projects\tryTen\private_comment\forms.py" in save
    15.             msg = userComment(buyer=buyer, seller=seller, comment=comment,)
  </code></pre>