# Example usage of database funtions(`Sum` and `Avg`) and Django `JSONField`'s `jsonb_array_length`

# PostgreSQL json field functions: https://www.postgresql.org/docs/9.5/static/functions-json.html
# Django `F` and `Func`: https://docs.djangoproject.com/en/1.10/ref/models/expressions/#f-expressions
# Django aggregation: https://docs.djangoproject.com/en/1.10/topics/db/aggregation/
# Django `JSONField`: https://docs.djangoproject.com/en/1.9/ref/contrib/postgres/fields/#jsonfield

# Given a model
class Contact(models.Model):
    numbers = JSONField(help_text="An array of numbers")

from django.db.models import Sum, Avg, F, Func

# Get the length of `numbers` field for one record
contact = Contact.objects.annotate(numbers_len=Func(F('numbers'), function='jsonb_array_length')).latest('pk')
>>> contact.numbers_len
3
# Get the sum of the length of all records
>>> contact = Contact.objects.annotate(numbers_len=Func(F('numbers'), function='jsonb_array_length')).aggregate(Sum('numbers_len'))
>>> contact
{'numbers_len__sum': 247485107}  # 247MM!
# Get the average length of all `numbers`
>>> contact = Contact.objects.annotate(numbers_len=Func(F('numbers'), function='jsonb_array_length')).aggregate(Avg('numbers_len'))
>>> contact
{'numbers_len__avg': Decimal('1.9999268527384372')}