Feature Suggestion for Migrations

[Calvin DeBoer]

Background

Django's current recommendation for adding data to a application is through the well documented "DataMigration" via RunSQL and RunPython calls. This represents an improvement over the fixture loading for a variety of reasons (it doesn't force developers to keep their fixtures up to date, control over overwriting data ,etc). Fixtures would be great if they didn't have to be kept up to date with the DB's current state (more on that later). As the applications we've been building with Django have grown in complexity we've seen an increase in the need to have control over which data is created in different application environments. For example, in production we want to add a large dataset foo (production data), but in development we only want to load a smaller dataset bar (for Developer Experience); furthermore, in tests we don't want to add ANY data because we want the tests, setUp method to granularly control what data is available in the DB with that set of tests. For the test piece, we investigated setting the MIGRATION_MODULES setting to None for all the apps in a test_settings file, but that seemed heavy handed, and also only handled a single application environment.

Suggestion

1. add a method called loaddata that takes a fixture and apps <- (the model context within a migration from which one can obtain a FakeModel instance representing the Model at state of that migration.

import os
from django.db import migrations
import environize

PATH = 'path/to/fixtures/'

def load_fixture(apps, schema_editor):
    fixture_file = os.path.join(PATH, 'myfixture.json')
    environize.loaddata(apps, fixture_file)


class Migration(migrations.Migration):

    dependencies = [
        ('app', '0003_auto_20180916_1122'),
    ]

    operations = [
        migrations.RunPython(load_fixture, lambda x, y: None)
    ]

2. allow RunPython and RunSQL to take an additional keyword that will tell django which envs to include or exclude in a DataMigration.

class Migration(migrations.Migration):

    dependencies = [
        ('app', '0002_auto_20180916_1122'),
    ]

    operations = [
        migrations.RunPython(add_prod_data, remove_hams, only_in=['production']),
        migrations.RunSQL("INSERT blah;", "REMOVE blah;", except_in=['test'])
    ]

Implementation

​https://github.com/cgdeboer/environize

Here is a hastily put together library to allow environments inside migrations and have state specific loaddata . The environment stuff is implemented as decorators on the function at the moment, probably makes more sense to just inherit RunPython and RunSQL and provide the keywords shows above in example 2.

Probably makes the most sense to continue to develop this as a package outside of Django, but before I do that, thought I'd see if there is any appetite for it in core django.