title: Django present related_name objects in admin model
body: <p>I have the following models:</p>
  
  <pre><code>class A(models.Model):
      name = models.CharField(max_length=100)
      c = models.ForeignKey(C, related_name='letters_c', null=True, blank=True)
      ...
  
  
  class B(models.Model):
      id= models.CharField(max_length=200, blank=False, default="")
      a = models.ForeignKey(A, related_name='letters_a', default=0)
      ...
  </code></pre>
  
  <p>With the following admin: </p>
  
  <pre><code>class AAdmin(admin.ModelAdmin):
      fields = ['name', 'c', 'letters_a']
      list_display = ['name']
      ordering = ['name']
  </code></pre>
  
  <p>I got an error </p>
  
  <pre><code>'letters_a' not found.
  </code></pre>
  
  <p>I guess I do not fully understand the logic between foreign keys and the representation of them in Django models.</p>
  
  <p>I want to be able to add/edit and see in the Django admin the class A <code>letters_a</code> related_name objects.</p>
  
  <p>How can I do it?</p>