Using GROUP_CONCAT with other annotations in Django - mysql

Question

I use an annotation which counts upvotes/downvotes while returning a list of articles:
queryset = queryset.annotate(
upvotes_count=models.Sum(
models.Case(
models.When(likes__like_state=1, then=1),
default=0,
output_field=models.IntegerField()
)
)
).annotate(
downvotes_count=models.Sum(
models.Case(
models.When(likes__like_state=-1, then=1),
default=0,
output_field=models.IntegerField()
))
)
But each article also has a few categories as ManyToMany related field and I needed to return those categories comma-separated, so I wrote this function:
class GroupConcat(models.Aggregate):
function = 'GROUP_CONCAT'
template = "%(function)s(%(distinct)s %(expressions)s %(separator)s)"
def __init__(self, expression, distinct=False, separator=', ', **extra):
super(GroupConcat, self).__init__(
expression,
distinct='DISTINCT' if distinct else '',
separator="SEPARATOR '%s'" % separator,
output_field=models.CharField(),
**extra
)
And added it to my annotation:
queryset = queryset.annotate(category=GroupConcat('categories__name'))
It works fine but upvotes_count and downvotes_count went crazy and started to multiply(!) results by amount of categories.
So the question is: "Is there a way to use GROUP_CONCAT in Django without breaking down SUM annotations?"

Answer 1

Very nice solution.
But to operate with group by field you should use order_by statement.
for example:
Store.objects.all().values('value').order_by('value').annotate(stores=GroupConcat('id'))
would generate sql statement
SELECT store.value, GROUP_CONCAT(store.id SEPARATOR ",") AS
stores FROM store WHERE store.value > 0 GROUP BY
store.value ORDER BY store.value ASC
and result would be
value, stores
1 "16,27"
Without order_by it would be like this:
SELECT store.value, GROUP_CONCAT(store.id SEPARATOR ",") AS
stores FROM store WHERE store.value > 0 GROUP BY store.id
ORDER BY store.value ASC
and result would be
value, stores
1 16
2 27