title: django ForeignKey get to_field
body: <p>If I make a ForeignKey definition in a model that has a to_field column definition is it possible to get that to_field value later?</p>
  
  <pre><code>class author(models.Model):
      name=models.CharField(max_length=255)
      customId=models.IntegerField(default=0, null=True, unique=True)
  
  class book(models.Model):
      name=models.CharField(max_length=255)
      author=models.ForeignKey(author,to_field='customId')
  </code></pre>
  
  <p>As shown above the foreignkey to author uses the <code>to_field</code> key, later on in my code I have to deal with book objects but would like to find out what field the author is linked to. </p>