gistfile1.py

class Observation(models.Model):
  invalid = models.BooleanField(default=False)

class Beam(models.Model):
  observation = models.ForeignKey(Observation)
  field = models.ForeignKey(Field)
  
class Field(models.Model):
  pass

# The aim: find all Fields for which all Beams belong to invalid Observations.

# This gives the correct answer:
fields = Field.objects.all().annotate(n_beams=Count('beam'))
fields = [f for f in fields if f.beam_set.filter(observation__invalid=True).count() == f.n_beams]

# ...but is woefully inefficient in terms of number of queries -- I see a query along the lines of this:
#
#SELECT COUNT(*) FROM "observationdb_beam" 
#  INNER JOIN "observationdb_observation" 
#    ON ("observationdb_beam"."observation_id" = "observationdb_observation"."obsid")
#  WHERE ("observationdb_beam"."field_id" = 669
#    AND "observationdb_observation"."invalid" = true
#
# for every field.  There must be a smarter way!