title: 'unique_together' refers to the non-existent field
body: <p>When trying to use <code>unique_together</code> on two foreign key constraints, I get the following error:</p>
  
  <pre><code>CommandError: System check identified some issues:
  
  ERRORS:
  main.AuthorTag: (models.E012) 'unique_together' refers to the non-existent field 'author'.
  main.AuthorTag: (models.E012) 'unique_together' refers to the non-existent field 'tag'.
  </code></pre>
  
  <p>On the following table</p>
  
  <pre><code>class AuthorTag(models.Model):
      class Meta:
          unique_together = (('tag', 'author',),)
      tag = models.ForeignKey(TagIndex, on_delete=models.CASCADE),
      author = models.ForeignKey(Author, on_delete=models.CASCADE),
  </code></pre>
  
  <p>The same example seems to work here: <a href="https://stackoverflow.com/questions/4522648/unique-foreign-key-pairs-with-django?rq=1">Unique foreign key pairs with Django</a>
  But I can't figure out why I get this error</p>
  
  <p>EDIT:
  changing <code>unique_together = (('tag', 'author',),)</code> to <code>unique_together = (('tag', 'author'))</code> give me the same error. as well as moving the meta class below the field declarations.</p>