migra/docs/with_django.md

Using migra with django

Incorporating non-default parts with django can be a little awkward. However, with some config tweaks you can use migra alongside django's default migration setup.

Below is a sample script that implements a hybrid sync command: It'll run any defined django migrations as normal, but allows you to specify a list of installed django "apps" that you can manage with migra instead.

Deploying

The general advice elsewhere in the docs regarding the creation of migration scripts for your production environment still applies as normal.

A script to generate production scripts would like quite similar the provided local-sync script below, except that the comparison would be production -> target instead of current local -> target.

Testing

Initialize your database for tests as follows:

• migrate any django migrations in use
• sync your prepared script of changes

A sample local syncing script

migra's built-in apps come with some migrations, so you don't want to disable the built-in migrations entirely.

:::python
import manage
import os
import sys
from mysite import settings
from django.core import management
import django
from sqlbag import temporary_database, S
from migra import Migration
from contextlib import contextmanager

# Point to your settings.
os.environ["DJANGO_SETTINGS_MODULE"] = "mysite.settings"

# The "polls" app (as per the django official tutorial)
# is set here to be managed with migra instead
MANAGE_WITH_MIGRA = ["polls"]

# To disable migrations on a django app, you have to
# set the MIGRATION_MODULES config with None for each
# disabled "app"
class DisableMigrations(object):
    def __contains__(self, item):
        return item in MANAGE_WITH_MIGRA

    def __getitem__(self, item):
        return None

# Compare two schemas, prompt to run a sync if necessary
def _sync_with_prompt(db_url_current, db_url_target):
    with S(db_url_current) as s0, S(db_url_target) as s1:
        m = Migration(s0, s1)
        m.set_safety(False)
        m.add_all_changes()

        if m.statements:
            print("THE FOLLOWING CHANGES ARE PENDING:", end="\n\n")
            print(m.sql)
            print()
            if input('Type "yes" to apply these changes: ') == "yes":
                print("Applying...")
                m.apply()
            else:
                print("Not applying.")
        else:
            print("Already synced.")


# Create a temporary database for loading our
# "target" schema into
@contextmanager
def tmptarget():
    with temporary_database() as tdb:
        settings.DATABASES["tmp_target"] = {
            "ENGINE": "django.db.backends.postgresql",
            "NAME": tdb.split("/")[-1],
        }
        django.setup()
        yield tdb


def syncdb():
    # Disable django migrations if we're using migra instead
    settings.MIGRATION_MODULES = DisableMigrations()


    with tmptarget() as tdb:
        management.call_command("migrate")

        management.call_command(
            "migrate",
            "--run-syncdb",
            "--database=tmp_target",
            verbosity=0,
            interactive=False,
        )
        real_db_name = settings.DATABASES["default"]["NAME"]
        _sync_with_prompt(f"postgresql:///{real_db_name}", tdb)


if __name__ == "__main__":
    syncdb()

Can it be done better?

How well does migra work for you with django? Let us know via email, github issues, twitter, whatever.