Django aggregating records for each foreign key - django

Question

class Order(models.Model):
product = models.ForeignKey(Product, on_delete=models.CASCADE)
category = models.ForeignKey(
Category, null=True, on_delete=models.SET_NULL
)
user = models.ForeignKey(User, null=True, on_delete=models.SET_NULL)
placed = models.DateTimeField(auto_now=True)
shipped = models.DateTimeField(null=True)
delivered = models.DateTimeField(null=True)
I want to calculate statistics on how fast the order has been processed for each category
where process time is delivered - shipped
In result I want to achieve something like this:
[
{
"category": <category 1>
"processed_time": <average processed time in seconds>
},
{
"category": <category 2>
"processed_time": <average processed time in seconds>
},
{
"category": <category 3>
"processed_time": <average processed time in seconds>
},
]
I can calculate this outside of the ORM but I'd like to achieve this somehow with annotation/aggregation
delivered = delivered_qs.annotate(first_processed=Min("delivered"), last_processed=Max("delivered")) \
.aggregate(processed_time=F("last_processed")-F("first_processed"))
This QS returns time only for all categories and I dont know how to retrieve time for each individual category

Answer 1

You want to do a group by, which in Django works kinda weird. For more information see the documentation
But by first using .values you say again the queryset you gonna group by on the category. Than you determine the min, the max and the difference.
delivered = (
delivered_qs
.values('category')
.annotate(
first_processed=Min("delivered"),
last_processed=Max("delivered"),
processed_time=F("last_processed") - F("first_processed"),
)
)
Which, in my expectation, would return:
[{
"category": 1,
"first_processed": timedelta(),
"last_processed": timedelta(),
"processed_time": timedelta()
}, ...]