optimise django sql query - django

Question

I'm using Django 2.x.
I have two models
class AmountGiven(models.Model):
contact = models.ForeignKey(Contact, on_delete=models.PROTECT)
amount = models.FloatField(help_text='Amount given to the contact')
interest_rate = models.FloatField(blank=True, default=None, null=True)
given_date = models.DateField(default=timezone.now)
total_due = models.FloatField(blank=True, default=0.0, editable=False)
class AmountReturned(models.Model):
amount_given = models.ForeignKey(AmountGiven, on_delete=models.CASCADE, blank=True)
amount = models.FloatField()
return_date = models.DateField(default=date.today)
Use case
There can be multiple records of the amount given to a contact
There can be multiple records of the returned amount for an amount given
Now, I want to get total_due amount for a particular contact. This includes
total_payable = total_given + interest
total_due = total_payable - total_returned
To calculate total_due and interest, I have defined few property methods in the AmountGiven model.
#property
def interest_to_pay(self):
if self.interest_rate:
simple_interest_amount = ...
return simple_interest_amount
return 0
#property
def total_payable(self):
return self.amount + self.interest_to_pay
#property
def amount_due(self):
total_due = self.total_payable - self.total_returned
self.total_due = total_due
self.save()
return total_due
#property
def total_returned(self):
returned_amount = self.amountreturned_set.aggregate(total_returned=Sum('amount'))['total_returned']
if not returned_amount:
returned_amount = 0
return returned_amount
In Contact model, there is a property method to get the total due amount for the contact.
#property
def amount_due(self):
total_due = 0
for due in self.amountgiven_set.all():
total_due += due.amount_due
return total_due
Query
ContactSerializer
class ContactMinSerializer(serializers.ModelSerializer):
class Meta:
model = Contact
fields = (
'id', 'first_name', 'amount_due', 'created', 'modified'
)
Since amount_due property is being used in the ContactSerializer, amount_due property is called everytime a contact is call and thus results in nested DB queries.
How can I optimise the above scenario in the application to reduce the DB queries while getting contact or list of contacts? Specially two properties amount_due and total_returned.
amount_due() updates the total_due field in the table, every time it is called.
Edit 2
class ContactViewSet(LoggingMixin, viewsets.ModelViewSet):
serializer_class = ContactMinSerializer
def get_queryset(self):
return Contact.objects.filter(user=self.request.user).annotate(
total_due=Sum(
F('amountgiven_set__total_payable')
- F('amountgiven_set__total_returned')
)
).order_by('first_name')

Answer 1

You're looking for annotations.
Your viewset should define a queryset as follows:
from django.db.models import F, Sum
Contact.objects.annotate(
total_due=Sum(
F('amountgiven_set__total_payable')
- F('amountgiven_set__total_returned')
)
)
Then define a MethodSerializer field on your serializer to account for it.
class ContactMinSerializer(serializers.ModelSerializer):
total_due = serializers.SerializerMethodField()
def get_total_due(self, obj):
return return self.total_due
class Meta:
model = Contact
fields = (
'id', 'first_name', 'created', 'modified',
'total_due',
)