Django queryset StringAgg on arrayfield - django

Question

I have some data which includes sizes, much like the model below.
class Product(models.Model):
width = models.CharField()
height = models.CharField()
length = models.CharField()
Through annotation we have a field called at_size which produces data like:
[None, None, None]
['200', '000', '210']
['180', None, None]
This was accomplished like so (thanks to: )https://stackoverflow.com/a/70266320/5731101:
class Array(Func):
template = '%(function)s[%(expressions)s]'
function = 'ARRAY'
out_format = ArrayField(CharField(max_length=200))
annotated_qs = Product.objects.all().annotate(
at_size=Array(F('width'), F('height'), F('length'),
output_field=out_format)
)
I'm trying to get this to convert into:
''
'200 x 000 x 210'
'180'
In code, this could a bit like ' x '.join([i for i in data if i]). But as I need to accomplish this with database functions it's a bit more challenging.
I've been playing with StringAgg, but I keep getting:
HINT: No function matches the given name and argument types. You might need to add explicit type casts.
It looks like I need to make sure the None values are excluded from the initial Array-func to begin with. But I'm not sure where to get started here.
How can I accomplish this?

Answer 1

Turns out the problem was two-fold.
Cleaning out the Null values could be done through using array_remove
glueing the strings with a delimiter through StringAgg only works if the input are strings. But since we use an array, that wasn't the way to go. Instead use array_to_string
The final result looks like:
class Array(Func):
# https://www.postgresql.org/docs/9.6/functions-array.html
template = '%(function)s[%(expressions)s]'
function = 'ARRAY'
class ArrayRemove(Func):
# https://www.postgresql.org/docs/9.6/functions-array.html
function = 'array_remove'
class ArrayToString(Func):
# https://stackoverflow.com/a/57873772/5731101
function = 'array_to_string'
out_format = ArrayField(CharField(max_length=200))
annotated_qs = annotated_qs.annotate(
at_size=ArrayToString(
ArrayRemove(
Array(F('width'), F('height'), F('length'), output_field=out_format),
None, # Remove None values from the Array with ArrayRemove
),
Value(" x "), # Delimiter.
Value(''), # If there are null-values, replace with... (fallback)
output_field=CharField(max_length=200),
)
)
this produces the desired format:
for product in annotated_qs:
print(product.at_size)
180 x 000 x 200
180 x 026 x 200
180 x 7 x 200
180 x 000 x 200
200 x 000 x 220
180 x 000 x 200
175 x 230 x 033
160 x 000 x 200
60 x 220

Answer 2

Product.objects.annotate(display_format=Concat(F('width'), '×', F('height'), '×', F('length')))
Should do the trick no?
No need to over complicate this, let's keep it nice and simple and use the database to concatenate the 3 strings and separate them with the multiplication symbol (obviously replace if you prefer another character).
Take a look at the docs over here:
https://docs.djangoproject.com/en/4.1/ref/models/database-functions/#concat