title: Django + South - migration can't cast the column type - wants me to use "USING"
body: <p>I'm getting this error when I try to run <code>./manage.py migrate my_app</code>:</p>
  
  <blockquote>
    <p>django.db.utils.ProgrammingError: column "associated" cannot be cast automatically to type integer<br>
    HINT:  Specify a USING expression to perform the conversion.</p>
  </blockquote>
  
  <p>That sounds like great advice, but I don't know exactly what <em>South</em> wants me to do.  Am I supposed to manually edit the auto-generated python migration file?  If so, how -- I don't see standard SQL in that file.  Instead, I see this:</p>
  
  <pre class="lang-python prettyprint-override"><code># Changing field 'MyTable.associated'
  db.alter_column(u'data_mytable', 'associated', 
      self.gf('django.db.models.fields.IntegerField')()
  )
  </code></pre>
  
  <p>Also, why didn't <em>South</em> pick up on this and add the <code>USING</code> clause automatically?  </p>