title: django updating foreignKey field before saving
body: <p>I have the following models.py</p>
  
  <pre><code>class Notebook(models.Model):
      category = models.ForeignKey(Category)
      title = models.CharField(max_length=200, help_text='Maximum 250 characters')
      slug = models.SlugField(unique=True)
      last_modified = models.DateTimeField()
  
      def __unicode__(self):
          return self.title
  
  
  class Page(models.Model):
      Notebook = models.ForeignKey(Notebook)
      title = models.CharField(max_length=200)
      slug = models.SlugField(unique_for_date='pub_date')
      tags = TaggableManager()
      pub_date = models.DateTimeField(default=datetime.datetime.now)
  
      def __unicode__(slef):
          reutrn self.title
  </code></pre>
  
  <p>As it is clear, a page belongs to a notebook and I have <code>last_modified</code> field in <code>Notebook</code> model.</p>
  
  <p>When I add a new page to my db, I want <code>last_modified</code> field updated to time of insert of the page.</p>
  
  <p>I know that I have to do some stuff by overrding the <code>save</code> method of <code>Page</code> class. But not sure of how to get fields from different class.</p>