Filter items with Django Query - django

Question

I'm encountering this problem and would like to seek your help.
The context:
I'm having a bag of balls, each of which has an age (red and blue) and color attributes.
What I want is to get the top 10 "youngest" balls and there are at most 3 blue balls (this means if there are more than 3 blue balls in the list of 10 youngest balls, then replace the "redudant" oldest blue balls with the youngest red balls)
To get top 10:
sel_balls = Ball.objects.all().sort('age')[:10]
Now, to also satisfy the conditions "at most 3 blue balls", I need to process further:
Iterate through sel_balls and count the number of blue balls (= B)
If B <= 3: do nothing
Else: get additional B - 3 red balls to replace the oldest (B - 3) blue balls (and these red balls must not have appeared in the original 10 balls already taken out). I figure I can do this by getting the oldest age value among the list of red balls and do another query like:
add_reds = Ball.objects.filter(age >= oldest_sel_age)[: B - 3]
My question is:
Is there any way that I can satisfy the constraints in only one query?
If I have to do 2 queries, is there any faster ways than the one method I mentioned above?
Thanks all.

Answer 1

Use Q for complex queries to the database: https://docs.djangoproject.com/en/dev/topics/db/queries/#complex-lookups-with-q-objects

Answer 2

You should use annotate to do it.
See documentation.
.filter() before .annotate() gives 'WHERE'
.filter() after .annotate() gives 'HAVING' (this is what you need)