title: Inline in Django admin: has no ForeignKey
body: <p>I have two Django models:</p>
  
  <pre class="lang-python prettyprint-override"><code>class Author(models.Model):
      user_email =  models.CharField(max_length=100, blank=True)
      display_name =  models.CharField(max_length=250)
  
  class Photo(models.Model):
      author = models.ForeignKey(Author)
      image = ThumbnailImageField(upload_to='photos')
  </code></pre>
  
  <p>To get inline photos, I have in admin.py:</p>
  
  <pre class="lang-python prettyprint-override"><code>class PhotoInline(admin.StackedInline):
      model = Author
  
  class AuthorAdmin(admin.ModelAdmin):
      list_display = ('display_name','user_email')
      inlines = [PhotoInline]
  </code></pre>
  
  <p>I get an error: <code>Exception at /admin/metainf/author/11/</code></p>
  
  <pre class="lang-none prettyprint-override"><code>&lt;class 'metainf.models.Author'&gt; has no ForeignKey to &lt;class 'metainf.models.Author'&gt;
  </code></pre>
  
  <p>Why?</p>