Django - save() update on duplicate key

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.

Answer 2

First, you must check if the rating exists. So you may either use what Daniel Roseman said or use exists, but you can not solve this with a simple update since update do not create new records...

rating = 2
rate, created = VideoRate.objects.get_or_create(user_id=1, video_id=1, crit_id=1,
    defaults={'rate':rating})#if create, also save the rate infdormation

if not created:# update
    rate.rate = rating
    rate.save()

You can use defaults to pass exrta arguments, so if it is an insert, database record will be created with all required information and you do not need to update it again...

Documentation

Update: This answer is quite old just like the question. As @peterthomassen mention, Django now have update_or_create() method