Get last instance object per each month and sum per month in Django

[Mario Bisiani]

I tryed to search for answer but nothing yet can help me.

I have one class model Activity and a class model Status :

class Status(models.Model):
    activity = models.ForeignKey(Activity, on_delete=models.CASCADE, related_name="all_status")
    status_date = models.DateField()
    actual_progress = models.DecimalField(max_digits=3, decimal_places=2)

I need to get last 'status_date' for each activity for each month and sum up its actual_progress.

Can't find after many research a way out.

Update

I found a way to get latest status per activity helped here Django - get latest object in each relation:

@classmethod
    def get_latest_by_activity_ids(cls, activity_id):
        found = []
        for aid in Activity.objects.all():
            found.append(cls.objects.filter(activity_id=aid).latest("status_date"))
        return found 

This gave me a list of dates. Now I'm blocked to sum them up per month.

https://stackoverflow.com/questions/60366457/get-last-instance-object-per-each-month-and-sum-per-month-in-django

[Agnese Camellini]

You should search how to make a group by activity and a partition by month then take each partition and sum the record up, i don't know how to do it in Django orm but in sql is the most effective way to solve the issue and it's a lot let expensive than taking all the activities and then search in the status Table.

my 2 cents

Agnese

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[Simon Charette]

If you don't want to operate on Activity instances annotations you can use the following queryset

Status.objects.annotate(

    status_month=TruncMonth('status_date'),

).order_by(

    'activity', 'status_month',

).values_list(

    'activity', 'status_month',

).annotate(

    progress_sum=Sum(actual_progress)

)

That will return an iterable of lists of the form [activity_id, status_month, progress_sum]

If you want to annotate your Activity instances you'll need to use window functions[0].

Simon

[0] https://docs.djangoproject.com/en/3.0/ref/models/expressions/#window-functions

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[Mario Bisiani]

Thank you a lot for your answer Simon! Your solution is very near to what I need. Next step to complete is to get latest date status per activity and then sum them up.  With your code I can get the list for every status. Thank a lot any way! I feel I'm next to the solution :-)

[Mario Bisiani]

Thank you Agnese , Grazie!

I will consider your prompt :-)

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