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

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...

CodePudding user response：

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

CodePudding user response：

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.

CodePudding user response：

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