I want to annotate MyModel queryset with a value from another History model. My models relations are the following:
class Stage(models.Model):
name = models.CharField()
class History(models.Model):
mymodel = models.ForeignKey(
MyModel,
on_delete=models.CASCADE,
)
stage = models.ForeignKey(
Stage,
on_delete=models.DO_NOTHING
)
created_at = models.DateTimeField(
auto_now_add=True
)
class MyModel(models.Model):
id = models.AutoField(
primary_key=True,
)
...
Approximate operation I'd like to perform in the ViewSet (order_by I need to get the latest stage):
class MyModelViewSet(viewsets.ModelViewSet):
...
def get_queryset(self):
qs = super(MyModelViewSet, self).get_queryset()
qs = qs.annotate(last_stage_name=Value(
History.objects.filter(mymodel__id=F('id')).order_by('-created_at')[0].stage.name,
output_field=CharField()))
But I get the same stage for all objects from the first object instead of the latest stage of every object, so the logics of operation is wrong. I suppose I should get rid of History.objects.filter but can't find appropriate solution.
You can work with a Subquery
expression [Django-doc] :
from django.db.models import OuterRef, Subquery
class MyModelViewSet(viewsets.ModelViewSet):
# …
def get_queryset(self):
return super().get_queryset().annotate(
last_stage_name=
History.objects.filter(
mymodel_id=
).order_by('-created_at').values('stage__name')[:1]
)
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