Django Model Field to Count Other Field

August 15, 2022

I have these models:

 
 class HatchingBatch(models.Model): 
     hatch_date = models.DateField() 
     incubator = models.CharField(max_length=32) 
     rhode_island_red = models.IntegerField(default=0) 
     barred_plymouth_rock = models.IntegerField(default=0) 
     black_australorp = models.IntegerField(default=0) 
      
 class Reservations(models.Model): 
     date = models.DateField() 
     hatch = models.ForeignKey(HatchingBatch, related_name="reservation", on_delete=models.CASCADE) 
     is_done = models.BooleanField() 
     is_delivery = models.BooleanField() 
     shipping_fee = models.IntegerField() 
     amount = models.IntegerField()  
  
 
 class Stocks(models.Model): 
     hatch = models.OneToOneField(HatchingBatch, on_delete=models.CASCADE, related_name="stock")

I want to add a field to the Stocks model that would be the number of the reserved chicks (easily done through ForeignKey) however, I also want to add a "sold" field that would correspond to the sum of the Reservations with "is_done" : True

>Solution :

You can .annotate(…) ^[Django-doc] with:

from django.db.models import Count, Q

Stocks.objects.annotate(
    reservations=Count('hatch__reservation'),
    sold=Count('hatch__reservation', filter=Q(hatch__reservation__is_done=True))
)

The Stocks objects that arise from this QuerySet will have an extra attributes .reservations and .sold.