Django ArrayAgg - filtering annotated array - django

Question

class Language(models.Model):
iso_code = models.CharField()
class Publisher(models.Model)
name = models.CharField()
class Book(modle.Model):
name = models.CharField()
language = models.ForeignKey(Language)
publisher = models.ForeignKey(Publisher, related_name='books')
lang_ids = [1,2]
qs = Publisher.objects.annotate(
x=ArrayAgg(
Case(
When(
books__language__in=lang_ids,
then="books__name"
)
)
)
)
I want to filter the qs as shown here - https://docs.djangoproject.com/en/3.1/ref/contrib/postgres/fields/#len
qs.filter(x__len=2)
Why is it impossible to filter the qs this way? I am getting an error IndexError: tuple index out of range.
Output field in ArrayAgg is ArrayField
class ArrayAgg(OrderableAggMixin, Aggregate):
function = 'ARRAY_AGG'
template = '%(function)s(%(distinct)s%(expressions)s %(ordering)s)'
allow_distinct = True
#property
def output_field(self):
return ArrayField(self.source_expressions[0].output_field)
def convert_value(self, value, expression, connection):
if not value:
return []
return value

Answer 1

In order to get length of an ArrayField, you will need to use a different function, there is cardinality and also array_length. More info here https://www.postgresql.org/docs/current/functions-array.html
If you have just one dimenssion array you can use cardinality
from django.db import models
class ArrayLength(models.Func):
function = 'CARDINALITY'
qs = Publisher.objects.annotate(
x=ArrayAgg(
Case(
When(
books__language__in=lang_ids,
then="books__name"
)
)
)
)
qs = qs.annotate(x_len=ArrayLength('x')) # notice `x_len` is just a name, you can use anything
qs = qs.filter(x_len=2) # the actual filter