Wandering Thoughts archives


My first Django database migration worked fine

We have a long standing Django application, using SQLite as the database layer because that's easy and all we need at our tiny scale. For as long as we've had the application I've not so much as breathed on its database schema (which is to say its model), because the thought of trying to do any sort of database migration was a bit scary. For reasons outside the scope of this entry, we recently decided that it was time to add some fields to our application model, so I got to (or had to) try out Django's support for more or less automatic database migrations. The whole experience was pretty painless in my simple case, although I had some learning experiences.

The basics of Django migrations are well covered by Django's documentation. For straightforward changes, you change your model(s) and then run the 'makemigrations' Django subcommand, supplying the name you want for your migration; Django will write a new Python file to do the work. Once you've made the migration you can then apply it with the 'migrate' subcommand, and in at least some cases un-apply it again. Our changes added simple fields that could be empty, which is about the simplest case you can get; I don't know how Django handles more complicated cases, for example introducing a mandatory non-null field.

One learning experience is that Django will want to create a additional new migrations if you tinker with the properties of your (new) model things after you've created (and probably applied) the initial migration in your development environment. For me, this happened relatively naturally as I was writing Django code to use these new fields, now that we had them. You probably don't want to wind up with a whole series of iterating migrations, so you're going to want to somehow squash these down to a single final migration for the production change. Since we use SQLite, I wound up just repeatedly removing the migration file and reverting my development database to start again, rather than tinker around with un-applying the migration and trying to get Django to rewrite it.

(Now that I'm reading the documentation all the way through there's a section on Squashing migrations, but it seems complex and not necessarily quite right for this case.)

While it's not strictly a migration issue as such, one thing that I initially forgot to do when I added new model fields was to also update the settings for our Django admin interface to display them. Partly this happened because it's been so long since I touched this code that I'd somewhat forgotten how it all worked until I looked at the admin interface, didn't see these fields in the particular model object, and started digging.

Although the Django migration documentation contains scary warnings about doing migrations with a SQLite database, I had no problem doing it in production with our generally barely used web application (we normally measure activity in 'hits per month'). Given how Django does database migrations on SQLite, a sufficiently active web application might have to shut down during the migration so that the database could be fiddled with without problems. In general, shutting down will avoid a situation where your running code is incompatible with the state of your database, which can definitely happen even in simple changes.

(Some people probably deploy such a database migration in multiple steps, but with a tiny application we did it the single step way, deploying a new version of the code that used the new fields at the same time as we performed the database migration on the production database.)

My overall experience is quite positive; changing our model was nowhere near as scary or involved as I expected it to be. I suspect I'm going to be much more willing to make model changes in the future, although I still don't think of it as a casual thing.

python/DjangoDBMigrationWentOkay written at 23:12:51; Add Comment

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