Speed up Django migration that adds two fields into one

Question

Currently trying to migrate two fields of information into one field. We have many, many objects to go through and it takes roughly 12 minutes to loop through all the objects. That would mean the website would be down for 12 minutes and we do not want that. Is there any way to speed up this migration? It works, it's just slow.

from __future__ import unicode_literals

from django.db import migrations, models


def set_people(apps, schema_editor):
    ParticipationCount = apps.get_model('partcount', 'ParticipationCount')
    for row in ParticipationCount.objects.all():
        row.people = row.male + row.female
        row.save()

class Migration(migrations.Migration):

    dependencies = [
        ('partcount', '0002_participationcount_people'),
    ]

    operations = [
        migrations.RunPython(set_people),
    ]

No is an acceptable answer in this case, I am just curious if there is a different way to do this, as loops can be slow. I am using Django 1.8 and python 2.

Answer 1

Unfortunately you're using django<2.2, since from 2.2 onwards you can use bulk_update , but you can still use something like

ParticipationCount.objects.all().update(
    people=F("male") + F("female")
)

Which will do it in one query to save time. This will fail for certain fields in Postgres however as it requires some specific type casting, but this can be done within the query