Question

I have little application which allows a user to rate a video.

The user can rate only once.
So I have defined the uniqueness on the model.

But he should be able change his rate.
So the save() should update on duplicate key

class VideoRate(models.Model):
  """Users can Rate each Video on the criterias defined for the topic"""
  user = models.ForeignKey(User)
  video = models.ForeignKey(VideoFile)
  crit = models.ForeignKey(VideoCrit)
  rate = models.DecimalField(max_digits=2, decimal_places=1, choices=RATE_CHOICES)
  class Meta:
    unique_together = (('user', 'video', 'crit'),)
    verbose_name = 'Video Rating'

If I

rate = VideoRate(user_id=1, video_id=1, crit_id=1, rate=2)
rate.save()

It’s saving the rating, but if I

rate = VideoRate(user_id=1, video_id=1, crit_id=1, rate=3)
rate.save()

I get the normal error

IntegrityError: (1062, "Duplicate entry '1-1-1' for key 'user_id'")

Even if I use force_update=True (since based only on primary keys)

Is there a way to update the rating if it already exists without having to check data before ?

Answer 1

To update an existing rating, you actually have to have the one you want to update. If you know the object may not exist, use get_or_create:

rate, created = VideoRate.objects.get_or_create(user_id=1, video_id=1, crit_id=1)
rate.rate = 2
rate.save()

You can short-cut the process by using update():

VideoRate.objects.filter(user_id=1, video_id=1, crit_id=1).update(rate=2)

But this will silently fail if the rating does not exist – it won’t create one.