title: Django South Migration Error: Key is Not Present
body: <p>I added these fields to my model:</p>
  
  <pre><code>class WatchList(models.Model):
      name = models.CharField(max_length=20)
  
  class Thing(models.Model):
      watchlist = models.ForeignKey(WatchList)
  </code></pre>
  
  <p>Ran the schemamigration successfully:</p>
  
  <pre><code> &gt;&gt;&gt; $ python2.7 manage.py schemamigration myapp --auto
   + Added model myapp.WatchList
   ? The field 'Thing.watchlist' does not have a default specified, yet is NOT NULL.
   ? Since you are adding this field, you MUST specify a default
   ? value to use for existing rows. Would you like to:
   ?  1. Quit now, and add a default to the field in models.py
   ?  2. Specify a one-off value to use for existing columns now
   ? Please select a choice: 2
   ? Please enter Python code for your one-off default value.
   ? The datetime module is available, so you can do e.g. datetime.date.today()
   &gt;&gt;&gt; 0
   + Added field watchlist on myapp.Thing
   Created 0004_auto__add_watchlist__add_field_thing_watchlist.py. You can now apply this migration with: ./manage.py migrate myapp
  </code></pre>
  
  <p>I haven't had a problem before trying to do this, but for some reason I received the following error:</p>
  
  <pre><code> &gt;&gt;&gt; $ python2.7 manage.py migrate myapp
  FATAL ERROR - The following SQL query failed: ALTER TABLE "myapp_thing" ADD CONSTRAINT "watchlist_id_refs_id_1b2eef756112b8e" FOREIGN KEY ("watchlist_id") REFERENCES "myapp_watchlist" ("id") DEFERRABLE INITIALLY DEFERRED;
  The error was: insert or update on table "myapp_thing" violates foreign key constraint "watchlist_id_refs_id_1b2eef756112b8e"
  DETAIL:  Key (watchlist_id)=(0) is not present in table "myapp_watchlist".
  </code></pre>
  
  <p>How can I successfully migrate the changes to the models? Thanks for any ideas that might help!</p>