Django ManyToManyField delete across models - django

Question

I've the below inefficient 'destroy' method for deleting Ratings that are held in Stimulus which itself is held within Experiment (I have simplified my models, for reasons of clarity).
Could you advise on a more efficient way of achieving this?
class Rating(models.Model):
rater = TextField(null=True)
rating = FloatField(null=True)
created = models.DateTimeField(null=True)
class Stimulus(TimeStampedModel):
genes = TextField()
weights = ListField()
ratings = ManyToManyField(Rating, null=True)
evaluation = FloatField(null=True)
complete = BooleanField(default=False)
Class Experiment(models.Model):
all_individuals = ManyToManyField(Stimulus, null=True)
def destroy(self):
all_ratings = Rating.objects.all()
for ind in self.all_individuals.all():
ratings = ind.ratings.all()
for rating in ratings:
if rating in all_ratings:
Rating.objects.filter(id = rating.id).delete()
Background: I am using Django to run an experiment (Experiment) which shows Users many Stimuli (Stimulus). Each Stimulus gets rated many times. Thus, I need to save multiple ratings per stimulus (and multiple stimuli per experiment).

Answer 1

Some simple improvements
Remove the if rating in all_ratings, every rating will be in the list of all ratings
Do the delete on the database side
ind.ratings.all().delete()
Use prefetch_related to get the foreign key objects
self.all_individuals.prefetch_related('ratings'):
Combined would be:
def destroy(self):
for ind in self.all_individuals.prefetch_related('ratings'):
ratings = ind.ratings.all().delete()

Answer 2

I think that in this case using ManyToManyField isn't the best choice.
You'll have less problems using common ForeignKey's changing a little the structure of this models.
Eg.
class Rating(models.Model):
rater = TextField(null=True)
rating = FloatField(null=True)
created = models.DateTimeField(null=True)
stimulus = models.ForeignKey('Stimulus', related_name='ratings')
class Stimulus(TimeStampedModel):
genes = TextField()
weights = ListField()
#ratings = ManyToManyField(Rating, null=True)
evaluation = FloatField(null=True)
complete = BooleanField(default=False)
experiment = models.ForeignKey('Experiment', related_name='stimulus')
class Experiment(models.Model):
#all_individuals = ManyToManyField(Stimulus, null=True)
name = models.CharField(max_length=100)
This is a more clear structure and when you delete Experiment by, experiment_instance.delete() a delete cascade will delete all other related models.
Hope it helps.