To keep it simple I have four tables(A, B, Category and Relation), Relation table stores the Intensity of A in B and Category stores the type of B.
A < Relation > B > Category
(So the relation between A and B is n to n, when the relation between B and Category is n to 1)
I need an ORM to group Relation records by Category and A, then calculate Sum of Intensity in each (Category, A) (seems simple till here), then I want to annotate Max of calculated Sum in each Category.
My code is something like:
A.objects.values('B_id').annotate(AcSum=Sum(Intensity)).annotate(Max(AcSum))
Which throws the error:
django.core.exceptions.FieldError: Cannot compute Max('AcSum'): 'AcSum' is an aggregate
Djangogroupby package with the same error.
For further information please also see this stackoverflow question.
I am using Django 2 and PostgreSQL.
Is there a way to achieve this using ORM, if there is not, what would be the solution using raw SQL expression?
Update
After lots of struggling I found out that what I wrote was indeed an aggregation, however what I want is to find out the maximum of AcSum of each A in each category. So I suppose I have to groupby the result once more after AcSum Calculation. Based on this insight I found a stackoverflow question which asks the same concept(The question was asked 1 year, 2 months ago without any accepted answer).
Chaining another values('id') to the set does not function neither as a group_by nor as a filter for output attributes, It removes AcSum from the set. Adding AcSum to values() is also not an option due to changes in the grouped by result set.
I think what I am trying to do is re grouping the grouped by query based on the fields inside a column (i.e id).
any thoughts?
