Django workaround to use window function call in an aggregate function? - django

Question

I'm trying to calculate customer order frequency.
First use a window function to get the previous order date then annotate the days since the last order.
from django.db.models import Avg, F, Window
from django.db.models.functions import ExtractDay, Lag, TruncDate
orders = (
Order.objects
.annotate(
prev_order_date=Window(
expression=Lag('paid_at', 1),
partition_by=[F('customer_email')],
order_by=F('paid_at').asc(),
),
days_since_last=ExtractDay(
TruncDate('paid_at') - TruncDate('prev_order_date')
),
)
)
Then group by customer_email before calculating the average frequency.
customer_data = (
orders.values('customer')
.annotate(avg_frequency=Avg('days_since_last'))
)
Unfortunately this throws an error. Does anyone know of a workaround or know of an alternate way to calculate the average frequency?
psycopg2.errors.GroupingError: aggregate function calls cannot contain window function calls

Answer 1

I found the django-cte package through this answer.
Join on the order id then make sure to annotate the result of the window function before grouping.
from django_cte import CTEManager, With
class OrderCTE(Order):
objects = CTEManager()
class Meta:
proxy = True
orders = With(
Order.objects
.annotate(
prev_order_date=Window(
expression=Lag('paid_at', 1),
partition_by=[F('customer_email')],
order_by=F('paid_at').asc(),
),
days_since_last=ExtractDay(
TruncDate('paid_at') - TruncDate('prev_order_date')
),
)
)
customer_data = list(
orders.join(OrderCTE, id=orders.col.id)
.with_cte(orders)
.annotate(days_since_last=orders.col.days_since_last)
.values('customer_email')
.order_by('customer_email')
.annotate(
avg_frequency=Avg('days_since_last'),
)
.values_list(
'customer_email',
'avg_frequency',
)
)