Django cross-relational query for custom model properties - mysql

Question

I wish to use Django's F to easily get cross-relational information. I had a working form of what I wanted before, but I felt it was ugly and so I transitioned to making custom properties for my models as so:
#property
def stage(self):
"""Returns the competition's stage status by date"""
now = timezone.now()
if self.end_date and self.pub_date and self.close_date and self.start_date:
if self.start_date > now:
return "Launched"
elif self.start_date < now < self.end_date:
return "Commenced"
elif self.end_date < now < self.close_date:
return "Scoring"
else:
return "Ended"
return "Inactive"
In the past, I used a huge When/Case block and annotated this in. I prefer 'property' style of doing things because their values don't mess up when aggregating. (Eg. Team counts get overcounted when I do sums etc. and so I have to do them separately)
However, now I get errors when I try to access: (Team->CompetitionTeam->Competition)
F('competitionteam__competition__stage')
Is there a way to do this? The errors are: (When debugging and trying to annotate the above)
django.core.exceptions.FieldError: Cannot resolve keyword 'stage' into field.
Just running straight from my code, it doesn't cough an error immediately after annotating, but only when the QuerySet in question is accessed.
django.core.exceptions.FieldError: Unsupported lookup 'stage' for AutoField or join on the field not permitted, perhaps you meant range?
Note*: There is no error if I am not annotating and just accessing them from the Objects directly.

Answer 1

You can't use F objects to access model properties because F objects are used when the query is accessing the database and the database does not have access to your python code.
My preference is to use annotations, which you said you stopped doing due to overcounting when you have multiple annotations. This happens because Django's default implementation of annotations uses joins instead of subqueries. This is noted in the docs on combining multiple aggregations.
I believe the solution here is to do the annotation using a subquery. The stage property doesn't use any relational data, so you should be able to do that as you were before. For the other annotations you are doing (you mention Team counts and sums) it's hard to show exactly what you need to do without seeing your models and what you want to annotate, but here are the docs on subquery.
You can also use the package django-sql-utils which provides aggregates like SubqueryCount and SubquerySum which work just like Django's built in Count and Sum but are implemented with subqueries instead of joins.