title: How I do save stars in Movie model?
body: <h1>Wha I want to achive.</h1>
  <p>I want to save stars in Movie model when def average_stars() is used.
  I want to put a number in stars and save it when the movie function is called like the code below. What should I do?</p>
  <pre><code>class Movie(models.Model):
      id = models.CharField(primary_key=True, editable=False,
                            validators=[alphanumeric],max_length = 9999)
      stars = models.FloatField(
                      blank=False,
                      null=False,
                      default=0, 
                      validators=[MinValueValidator(0.0),
                       MaxValueValidator(10.0)]
                       )
      def get_comments(self):
          return Comment_movie.objects.filter(movie_id=self.id)
      
      def average_stars(self):
          comments = self.get_comments()
          n_comments = comments.count()
  
          if n_comments:
              self.stars = sum([comment.stars for comment in comments]) / n_comments
          else:
              self.stars = 0
          return self.stars
  </code></pre>