django aggregate sum of subgroup maximums - django

Question

I have the following simplified setup in Django, using MySQL:
class Category(models.Model):
name = models.CharField(max_length=255)
class MyTable(models.Model):
category = models.ForeignKey('Category', on_delete=models.CASCADE)
date = models.DateField()
amount = models.IntegerField()
class Meta:
unique_together = ['category', 'date']
And a query set comprised of the following sample data for MyTable:
date category_id amount
2017-12-01 3 2
2018-01-01 1 100
2018-02-01 1 50
2018-03-01 2 2000
2018-04-01 2 4000
2018-05-01 3 2
2018-06-01 3 1
What I ultimately want is a way to get the sum of the amounts corresponding to the latest date for each category. To illustrate:
The latest date for category_id 1 is 2018-02-01, where the amount is 50;
The latest date for category_id 2 is 2018-04-01, where the amount is 4000;
The latest date for category_id 3 is 2018-06-01, where the amount is 1;
50 + 4000 + 1 = 4051
I'm trying to figure how to get the value 4051 through an aggregate call to this queryset. I've tried every combination of "values", "annotate" and "aggregate" I could think of and nothing gets the desired result. The following gets me the latest date for each category, but every time I try to get the sum of the corresponding amounts, it calculates it on every amount instead of just the maximums.
MyTable.objects.values('category').annotate(Max('date'))
Is what I'm trying to do possible through Django's ORM? I posted another question about what the MySQL syntax would be for this exact example, but can't get that to apply to a Django query set either.
Any guidance appreciated.

Answer 1

i think you need some data structures to achieve this please take a look at the below answer this if Djnago ORM or sql does not meet your requirement(unable to write)
l=[]
x=MyTable.objects.values('category').annotate(date=Max('date'))
for y in x:
qq= MyTable.objects.get(category_id=y.get(category),date=y.get(date))
l=l+qq.amount
total_amount = sum(l)
print(total_amount)