Wrangling Django ArrayField Migrations

Unfortunately you can't depend on makemigrations to generate the correct SQL to migrate and cast data from a scalar field to a PostgreSQL ARRAY. But Django provides a nifty RunSQL that's also described in this post, "Down and Dirty - 9/25/2013" by Aeracode, the original creator of South predecessor of Django migrations. That post even mentions using RunSQL to alter a column using CAST.

The issue and trick to migrating a column to an ArrayField is given by PostgreSQL in the traceback, which says:

column "my_field " cannot be cast automatically to type double precision[]
HINT:  Specify a USING expression to perform the conversion.

Further hints can be found by rtfm and searching the internet, such this stackoverflow Q&A. My procedure was to use makemigrations to get the state_operations and then wrap each one into a RunSQL migration operation.

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import migrations, models
import datetime
from django.utils.timezone import utc
import django.contrib.postgres.fields
import simengapi_app.models
import django.core.validators


class Migration(migrations.Migration):

    dependencies = [
        ('my_app', '0XYZ_auto_YYYYMMDD_hhmm'),
    ]

    operations = [
        migrations.RunSQL(
            """
            ALTER TABLE my_app_mymodel
            ALTER COLUMN "my_field"
            TYPE double precision[]
            USING array["my_field"]::double precision[];
            """,
            state_operations=[
                migrations.AlterField(
                    model_name='mymodel',
                    name='my_field',
                    field=django.contrib.postgres.fields.ArrayField(
                        base_field=models.FloatField(), default=list,
                        verbose_name=b'my field', size=None
                    ),
                )
            ],
        ),
    ]