Django : Recalculating mean value in the database after creating a new instance - django

Question

I have informations about companies presented in a table. One of the field of this table is the mean value of each note the company received ('note_moyenne' in models.FicheIdentification).
By clicking on a button, people are able to submit a new note for the company ('note' in models.EvaluationGenerale). I want the mean value of the notes to update in the database each time someone submit a new note.
Here is my models.py :
class FicheIdentification(models.Model):
entreprise=models.ForeignKey(Entreprise, on_delete=models.CASCADE)
note_moyenne=models.IntegerField()
def __str__(self):
return self.entreprise.nom_entreprise
class EvaluationGenerale(models.Model):
entreprise=models.ForeignKey(Entreprise, on_delete=models.CASCADE)
note=models.IntegerField()
commentaires=models.CharField(max_length=1000)
date_evaluation=models.DateField(auto_now_add=True)
def __str__(self):
return self.commentaires
views.py :
class CreerEvaluationGenerale(CreateView):
form_class = FormulaireEvaluationGenerale
model = EvaluationGenerale
def form_valid(self, form):
form.instance.entreprise=Entreprise.objects.filter(siret=self.kwargs['siret']).first()
return super(CreerEvaluationGenerale, self).form_valid(form)
def get_success_url(self):
return reverse('details-evaluations')
Currently I just display the mean value in my table using this
def render_evaluation(self, record):
return (EvaluationGenerale.objects.filter(entreprise=record.entreprise.siret).aggregate(Avg('note'))['note__avg'])
but I really don't like this solution as I want the value to be stored in the database, in FicheIdentification.note_moyenne.
I thought about creating a UpdateView class but couldn't manage to link it with my CreateView.
Any help or documentation would be really appreciated, I'm a bit lost right know...

Answer 1

Typically, you would not store calculated fields. The usual way is not to store the average, but to use an annotation/aggregation in your query.
To centralize this to your model, you would want to write a custom model manager to implement this, so it can be reused anywhere you use your model without rewriting the logic.
class MyModelManager(models.Manager):
def note_average(self, **filter_kwargs):
qs = self.get_queryset()
# replace `...` with your aggregation as needed
return qs.filter(**filter_kwargs).aggregate(...)
class EvaluationGenerale(models.Model):
objects = MyModelManager() # override the default manager
# ... the rest of the model as-is
Then you can use something like the following in your view(s):
EvaluationGenerale.objects.note_average(entreprise=record.entreprise.siret)
See for additional reference: How to add a calculated field to a Django model

Answer 2

I see two ways of doing it.
Either a listener post_save on EvaluationGenerale (doc). You'll be able to compute the new average each time a new EvaluationGenerale is entered in DB.
#receiver(post_save, sender=EvaluationGenerale)
def evaluation_general_note_moyenne_computer_post_save_listener(sender, instance, **kwargs):
entreprise = instance.entreprise
entreprise.note_moyenne = entreprise.evaluationgeneral_set.aggregate(Avg('note')).values()[0])
entreprise.save()
post save listener will only trigger on instance.save() and models.objects.create() not on queryset.update() or model.objects.bulk_create().
Either overriding the save (doc) function of your form to compute the average after the creation of the new EvaluationGenerale
def save(self):
instance = super.save()
entreprise = instance.entreprise
entreprise.note_moyenne = entreprise.evaluationgeneral_set.aggregate(Avg('note')).values()[0]
entreprise.save()
return instance

Answer 3

Assuming there is as single FicheIdentification object per enterprise, you could update the note_moyenne field when you save the EvaluationGenerale object, like:
obj = FicheIdentification(...)
FicheIdentification.objects.filter(entreprise=record.entreprise.siret).update(note_moyenne=obj.aggregate(Avg('note'))['note__avg']
obj.save()
Please let me know if it works.