How to use StringAgg aggregation functions in PostgreSQL 9.6 using DjangoORM

Question

I am trying to join a field over a group. I could handle it in MySQL as described in one of my previous questions. However I migrated to PostgreSQL now and the proposed solutions does not work in PostgreSQL 9.6. According to Django docs, it is possible to use StringAgg as described here or here. I believe, in newer versions of PostgreSQL I cannot execute the line:

from django.db.models.sql.aggregates import Aggregate as SQLAggregate

Which throws the error:

from django.db.models.sql.aggregates import Aggregate as SQLAggregate
ModuleNotFoundError: No module named 'django.db.models.sql.aggregates'

How can I create my own Aggregate Function using StringAgg?

Update

It seems I dodn't need to modify StringAgg to calculate what I needed. I just imported it as Exprator described in their answer:

from django.contrib.postgres.aggregates import StringAgg

and used it along with values() to group by the query. Since the fields were not string I had to use Cast as well:

from django.contrib.postgres.aggregates import StringAgg
from django.db.models.functions import Cast
from django.db.models import TextField

query.annotate(
        AggregatedType = StringAgg(Cast('Types', TextField()),delimiter=',')
    )

Do you try the exists string_agg of PostgreSQL? The document is here https://www.postgresql.org/docs/current/static/functions-aggregate.html — Mabu Kloesen, Jan 20 '18 at 06:56
Read: https://learnbatta.com/blog/usage-of-group_by-string_agg-in-django-postgresql-74/ — anjaneyulubatta505, Jul 10 '18 at 18:43

score 10 · Accepted Answer · answered Jan 20 '18 at 07:04

10

from django.contrib.postgres.aggregates import StringAgg

this is how you can import the aggregate functions

answered Jan 20 '18 at 07:04

Exprator

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Thank you for the answer. I successfully imported StringAgg. Now I'm on creating custom aggregation function task. – Azee Jan 20 '18 at 07:51
the final answer could be found in the update of the question. – Azee Jan 20 '18 at 09:20
is there any way to replicate the functioning of StringAgg on MariaDB ? – Skratt Sep 02 '21 at 08:37

How to use StringAgg aggregation functions in PostgreSQL 9.6 using DjangoORM

Update

1 Answers1