title: How to migrate data from a field to another in a table using south
body: <p>Im working on a Django project using south for schema migrations.
  I have the following scenario:</p>
  
  <h2>schema</h2>
  
  <pre><code>class Book(model.Models):
      name = models.CharField(max_length=255, blank=True)
      bid = models.IntegerField(blank=True, null=True)
  
  class Author(model.Models):
     name = models.CharField(max_length=255, blank=True)
     book_id = models.ForeignKey(Book, null=True, to_field="bid", db_column="bookID")
  </code></pre>
  
  <p>I wanna change Author model to the following:</p>
  
  <pre><code>class Author(model.Models):
     name = models.CharField(max_length=255, blank=True)
     book = models.ForeignKey(Book, null=True, db_column="book_id")
  </code></pre>
  
  <p>but without loose data. I want to search each book by its bid and assign the one found to the new field in Author model.</p>
  
  <p>Thanks</p>