title: Migrate Django model to unique_together constraint
body: <p>I have a model with three fields</p>
  
  <pre><code>class MyModel(models.Model):
      a    = models.ForeignKey(A)
      b    = models.ForeignKey(B)
      c    = models.ForeignKey(C)
  </code></pre>
  
  <p>I want to enforce a unique constraint between these fields, and found django's <code>unique_together</code>, which seems to be the solution. However, I already have an existing database, and there are many duplicates. I know that since <code>unique_together</code> works at the database level, I need to unique-ify the rows, and then try a migration.</p>
  
  <p>Is there a good way to go about removing duplicates (where a duplicate has the same (A,B,C)) so that I can run migration to get the <code>unique_together</code> contstraint?</p>