title: Cannot assign must be a instance Django
body: <p>I have an order form which returns this statement of submit:</p>
  
  <pre><code>Cannot assign "&lt;Annual: 2012&gt;": "Order.annuals" must be a "Catalog" instance.
  </code></pre>
  
  <p>I'm fairly new to Django. I understand it needs an instance instead of the string it has been passed. How would I go about resolving that?</p>
  
  <p>Here is my view:</p>
  
  <pre><code>class OrderListCreateView(
    views.LoginRequiredMixin,
    views.SetHeadlineMixin,
    generic.CreateView
       ):
    form_class = forms.OrderListForm
    headline = 'Create'
    model = Order
    template_name = 'ordercreate.html'
  
  
    def form_valid(self, form):
      self.object = form.save(commit=False)
      self.object.user = self.request.user
      self.object.save()
      return super(OrderListCreateView, self).form_valid(form)
  </code></pre>
  
  <p>Here is my form:</p>
  
  <pre><code>class OrderListForm(forms.ModelForm):
  
  annuals = forms.ModelChoiceField(queryset=Annual.objects.all())
  issues = forms.ModelChoiceField(queryset=Issue.objects.all())
  articles = forms.ModelChoiceField(queryset=Article.objects.all())
  
  class Meta:
      fields = (
                  'annuals',
                  'issues',
                  'articles',)
      model = models.Order
  
  def __init__(self, *args, **kwargs):
      super(OrderListForm, self).__init__(*args, **kwargs)
      self.helper = FormHelper()
      self.helper.layout = Layout(
                  'annuals',
                  'issues',
                  'articles',
          ButtonHolder(
              Submit('create', 'Create')
  
          )
  
      )
  </code></pre>
  
  <p>Here is my model:</p>
  
  <pre><code>class Catalog(models.Model):
      products = models.CharField(max_length=200)
  
      def __unicode__(self):
          return self.products
  
  
  class Issue(models.Model):
      catalog = models.ForeignKey(Catalog, related_name='issue_products')
      Volume = models.DecimalField(max_digits=3, decimal_places=1)
  
      def __unicode__(self):
          return unicode(self.Volume)
  
  
  class Annual(models.Model):
      catalog = models.ForeignKey(Catalog, related_name='annual_products')
      year_id = models.IntegerField(max_length=4)
      start_date = models.CharField(max_length=6)
      end_date = models.CharField(max_length=6)
      def __unicode__(self):
          return unicode(self.year_id)
  
      #def __unicode__(self):
      #    return unicode(self.id)
  
  class Annual_Issue(models.Model):
      annual_id = models.ForeignKey(Annual, related_name='annual_ids')
      issue_id = models.ForeignKey(Issue, related_name='issues')
      def __unicode__(self):
          return self.annual_id
  
  
  class Article(models.Model):
      catalog = models.ForeignKey(Catalog, related_name='article_products')
      title = models.CharField(max_length=200)
      abstract = models.TextField(max_length=1000, blank=True)
      full_text = models.TextField(blank=True)
      proquest_link = models.CharField(max_length=200, blank=True, null=True)
      ebsco_link = models.CharField(max_length=200, blank=True, null=True)
  
      def __unicode__(self):
          return self.title
  
  
  class Order(models.Model):
      user = models.ForeignKey(User, related_name='who_ordered')
      annuals = models.ForeignKey(Catalog, related_name='annuals_ordered', blank=True, null=True)
      issues = models.ForeignKey(Catalog, related_name='issues_ordered', blank=True, null=True)
      articles = models.ForeignKey(Catalog, related_name='items_ordered', blank=True, null=True)
  </code></pre>