Django 1.11.4 Postgresql SELECT ARRAY to django orm

Question

I am trying to build a complex(for me) query for one of my projects. Django version is 1.11.4 and PostgreSQL version is 9.6.

Here are the models.

class Event(models.Model):
    ...
    name = models.CharField(max_length=256)
    classification = models.ForeignKey("events.Classification", related_name="events", null=True, blank=True)
    ...

class Classification(models.Model):
    ...
    segment = models.ForeignKey("events.ClassificationSegment", related_name="classifications", blank=True, null=True)
    ...

class ClassificationSegment(models.Model):
    ...
    name = models.CharField(max_length=256)
    ...

I blocked somewhere here and can't go ahead.

from django.db.models import CharField, Value as V
from django.db.models.functions import Concat
from django.contrib.postgres.aggregates import ArrayAgg
from django.db.models import OuterRef, Subquery
import events.models


event_subquery = events.models.Event.objects.filter(classification__segment=OuterRef('pk')) \
.annotate(event=Concat(V('{id:'), 'id', V(', name:"'), 'name', V('"}'), output_field=CharField()))

final_list = events.models.ClassificationSegment.objects.annotate(
event_list=ArrayAgg(Subquery(event_subquery.values('event')[:6])))

I have a raw query. Here it is.

final_events = events.models.ClassificationSegment.objects.raw('SELECT "events_classificationsegment"."id", "events_classificationsegment"."name", (SELECT ARRAY(SELECT CONCAT(\'{id:\', CONCAT(U0."id", CONCAT(\',\', \'name:"\', U0."name", \'"}\'))) AS "event" FROM "events_event" U0 INNER JOIN "events_classification" U1 ON (U0."classification_id" = U1."id") WHERE U1."segment_id" = ("events_classificationsegment"."id") LIMIT 6)) AS "event_list" FROM "events_classificationsegment"')

You can see the result in the screenshot. I guess I am on the right way. Can anyone help me?

Thanks.

Answer 1

Postgres has a really nice way of making an array from a subquery:

SELECT foo.id, ARRAY(SELECT bar FROM baz WHERE foo_id = foo.id) AS bars
  FROM foo

To do this within the ORM, you can define a subclass of Subquery:

class Array(Subquery):
    template = 'ARRAY(%(subquery)s)'

and use this in your queryset:

queryset = ClassificationSegment.objects.annotate(
    event_list=Array(event_subquery.values('event')[:6])
)