title: django-tastypie foreign key trouble
body: <p>I have a post model and a postImage model:</p>
  
  <pre><code>class PostImage(models.Model):
      post =  models.ForeignKey(Post, related_name="images")
      # @@@@ figure out a way to have image folders per user...
      image = models.ImageField(upload_to='images')
      description = models.CharField(max_length=100)
      order = models.IntegerField()
  </code></pre>
  
  <p>I have created tastypie resources for both:</p>
  
  <pre><code>class PostImageResource(ModelResource):
      class Meta:
          queryset = PostImage.objects.all()
          resource_name = 'postImage'
  
  
  class PostResource(ModelResource):
      images = fields.ForeignKey(PostImageResource, 'images', full=True)
      class Meta:
          queryset = Post.objects.all()
          resource_name = 'post'
  </code></pre>
  
  <p>When I try to browse to <code>api/v1/post/?format=json</code> I get the following error:</p>
  
  <pre><code>The object '' has an empty attribute 'image' and doesn't allow a default or null value.
  </code></pre>
  
  <p><strong>I don't have any PostImages yet.</strong></p>
  
  <p>If I get rid of the <code>images = fields.foreignKey</code> line in <code>PostResource</code>, it works. If I browse to <code>api/v1/postImage/?format=json</code>, no error is shown, just an empty collection.</p>