How to have a field represent the count of related objects - django

Question

I have a couple of Django models, Channel and Recording. Channels have a one-to-many relationship with recordings, such that a channel may have many recordings. Within the Channel model, I currently have 2 fields, num_recordings, and storage_size, which are right now created as Integers, but which I want to have dynamically generated as the result of a DB query. E.g. num_recordings should be the current count of recordings on a given channel, and storage_size should be the sum of a field "size" for each of the recordings on a given channel.
I'm using Django rest framework to give a JSON representation of the models, and want it so that when I query the Channel, I see these two fields as Integers, but don't want to have to calculate these separately, e.g. it would be ideal if when you query the Channel endpoint, it would perform a count on the recording relationship, and return that as "num_recordings" and a sum on the recording.size field for all recordings in the relationship and report that on the storage_size field.
How can I do this?

Answer 1

You can approach it by benefiting from the related_name attribute of ForeignKey fields in Django and standard Python #property decorator.
At first, let's assume that you have defined your relationship in models.py like this:
class Channel(models.Model):
...
class Recording(models.Model):
channel = models.ForeignKey(Channel, on_delete=..., related_name='recordings')
# ________________________________________________________________^
# This is the reverse relation name, e.g. channel.recordings
From now on, you can access the related recordings of a specific channel from the channel itself:
>>> channel = Channel.objects.create(...)
>>> recording1 = Recording.objects.create(channel=channel, ...)
>>> recording2 = Recording.objects.create(channel=channel, ...)
>>> channel.recordings.all()
<QuerySet [<Recording: 1>, <Recording: 2>]>
>>> channel.recordings.count()
2
You can use those methods in your Channel model directly:
class Channel(models.Model):
...
#property
def num_of_recordings(self):
return self.recordings.count()
# __________^ (related_name)
Now for your storage_size, you can aggregate it via the aggregate() method of QuerySet and the Sum aggregation function:
from django.db.models import Sum
class Channel(models.Model):
...
#property
def storage_size(self):
return self.recordings.aggregate(storage_size=Sum('size'))['storage_size']
And now the final Channel model:
class Channel(models.Model):
...
#property
def num_of_recordings(self):
return self.recordings.count()
#property
def storage_size(self):
return self.recordings.aggregate(storage_size=Sum('size'))['storage_size']
The last step is to add these new properties num_of_recordings and storage_size in your channel serializer class in order to display them. Since they are decorated with #property, they are read-only by design and thus are calculated dynamically based on the related recordings.