How to create an annotation in Django that references two related models - django

Question

I'm trying to add an annotation to a QuerySet that is True/False when the value of a field on one related object is less than the value of a field on a different related object.
Here are some models for an example:
class RobotManager(models.Manager):
queryset = super(RobotManager, self).get_queryset()
queryset = queryset.annotate(canteen_empty=UNKNOWN CODE)
return queryset
class Robot(models.Model):
# Has some other unrelated stuff
objects = RobotManager()
class CanteenLevel(models.Model):
time = models.DateTimeField()
robot = models.ForeignKey("SomeApp.Robot")
gallons = models.IntegerField()
class RobotConfiguration(models.Model):
time = models.DateTimeField()
robot = models.ForeignKey("SomeApp.Robot")
canteen_empty_level = models.IntegerField()
With the above models, as the Robot's Configuration or CanteenLevel change, we create new records and save the historicals.
What I would like to do is add an annotation to a Robot QuerySet that states if the Robot's Canteen is considered empty (Robot's latest CanteenLevel.gallons is less than the Robot's latest Configuration.canteen_empty_level).
The aim is to allow for a statement like this using the annotation in the QuerySet:
bad_robots = Robot.objects.filter(canteen_empty=True)
I had tried something like this in the annotation:
canteen_empty=ExpressionWrapper(CanteenLevel.objects.filter(robot=OuterRef('pk')).order_by('-time').values('gallons')[:1] <= RobotConfiguration.objects.filter(robot=OuterRef('robot')).order_by('-time').values('canteen_empty_level')[:1], output_field=models.BooleanField))
But obviously the "<=" operator isn't allowed.
I also tried this:
canteen_empty=Exists(CanteenLevel.objects.filter(robot=OuterRef('pk')).order_by('-time').values('gallons')[:1].filter(gallons__lte=Subquery(RobotConfiguration.objects.filter(robot=OuterRef('robot')).order_by('-time').values('canteen_empty_level')[:1]))))
But you can't filter after taking a slice of a QuerySet.
Any help would be appreciated!

Answer 1

We can make two annotations here:
from django.db.models import Subquery, OuterRef
latest_gallons = Subquery(CanteenLevel.objects.filter(
robot=OuterRef('pk')
).order_by('-time').values('gallons')[:1])
latest_canteen = Subquery(RobotConfiguration.objects.filter(
robot=OuterRef('pk')
).order_by('-time').values('canteen_empty_level')[:1])
then we can first annotate the Robot objects with these, and filter:
from django.db.models import F
Robot.objects.annotate(
latest_gallons=latest_gallons,
latest_canteen=latest_canteen
).filter(latest_gallons__lte=F('latest_canteen'))
This will construct a query that looks like:
SELECT robot.*,
(SELECT U0.gallons
FROM canteenlevel U0
WHERE U0.robot_id = robot.id
ORDER BY U0.time DESC
LIMIT 1) AS latest_gallons,
(SELECT U0.canteen_empty_level
FROM robotconfiguration U0
WHERE U0.robot_id = robot.id
ORDER BY U0.time DESC
LIMIT 1) AS latest_canteen
FROM robot
WHERE
(SELECT U0.gallons
FROM canteenlevel U0
WHERE U0.robot_id = robot.id
ORDER BY U0.time DESC
LIMIT 1
) <= (
SELECT U0.canteen_empty_level
FROM robotconfiguration U0
WHERE U0.robot_id = robot.id
ORDER BY U0.time DESC
LIMIT 1
)
Note however that if a Robot has no related CanteenLevel, or RobotConfiguration (one of them, or both), then that Robot will not be included in the queryset.