Tag Archives: migration

Migrating Django Migrations to Django 2.x

Django is a Python framework for making web applications, and its impressive in its completeness, flexibility and power for speedy prototyping.

It’s also an impressive project for forward planning, it has a kind of built in “lint” functionality that warns about deprecated code that will be disallowed in future versions.

As a result when Django 2.0 was released I didn’t have to make many changes to my app code base to get it to work successfully. However, today when I tried to update my oldest Django App (started in Django 1.8x) I hit an unexpected snag. The old migrations were sometimes invalid. Curiously I don’t think this problem emerged the last time I tried.

Django uses migrations to move the database schema from one version to the next. Most of the time it’s a wonderful system. In the rare case it goes wrong it can be … tricky. Today’s problem is quite specific, and easier to fix.

Django 2.0 enforces that ForeignKey fields explicitly specify a behaviour to follow on deletion of the object pointed to by the key. In general whether we Cascade the deletion, or set the field to Null, getting the behaviour write can be important, particular on fields where a Null value has a legitimate meaning.

But a bit of a sting in the tail is that an older Django project may have migrations created automatically by Django which don’t obey this. I discovered this today and found I couldn’t proceed with my project unless I went back and modified the old migrations to be 2.0 compliant.

So if this happens to you, here are some suggestions on fixing the problem.

You will know if you have a problem if when you try to run your test server, or indeed replace runserver by check

you get an error and output like this

I would suggest you try runserver whatever you did before as it will continue to try each time you save a file.

Open your code with your favourite editor, and open your models.py file (you may have several depending on your project), and the migration file that’s broken as above.

Looking in your migration file you’ll find the offending line. In this case it’s the last (non trivial) line below.

To ensure that your migrations will be applied consistently with your final model (well, as long as nobody tries to migrate to an intermediate state) look carefully in the correct model (Activity) in this case, and see what decision you make for deletion there. In my case I want deletion of the ActivitySet to kill all linked Activitiy(s). So replicate the “on_delete” choice from there.

Each time you save your new migration file the runserver terminal window will re-run the check, hopefully moving on to the next migration that needs to be fixed. Work your way through methodically until your code checks clean. Check into source control, and you’re done.

 

Manually completing a botched django migration

I wrote a lot of code for my Workload Allocation system on Friday, and had been developing it on the machine with django’s built in lightweight web server, and a (default) sqlite database backend. In production I decided to use a MySQL backend in case sqlite was, well, too lite.

One of the things that is really neat about django, but which also profoundly scares me, is that it handles changes to the database schema automatically. I am used to doing all of this by hand. It has been a pleasant change, but I wondered what would happen if it went wrong.

Which it did on Friday. The migrations had worked perfectly well on the development server and after some testing I decided to roll the code into production, whereupon the migration failed. I’m still not sure why, but something in the django deep magic failed. To make things worse the process is, I have discovered, not idempotent, and trying to run the migration again caused it to fail in new places because some of the database schema changes had been successful; so it was now bailing out with “already exists” kind of errors.

Removing some tables and trying again didn’t quite do the trick. I thought about trying to fix the schema manually, since with the mysql command line tool I could see what fields needed to be added, but upon inspection the restraints added by django were complex and I was unsure how important they were.

So this is my clumsy workaround, that will no doubt come back to haunt me.

I used the following commands from the top of the django app directory to find the name of the migration that was failing, and than used –fake to force django to forget about having to apply it.

I then created a “manual” django migration that added the new fields.

It turns out that getting the dependency right at the top is very important, it needs to be previous migration.

The name of this script is important, follow the naming convention of your most recent failed migration, changing auto to custom and the timestamp appropriately. I discovered that django, would not run this migration. It detected a conflict with the previous migration that should have created the fields and wanted me to try and merge them. That would be pointless since the previous migration failed. I also discovered to my surprise there was no –force command line switch to override this logic, though Google perhaps suggests that previous versions of django allowed this.

So, I used the sqlmigration django command to output the correct SQL that it would produce if this migration did run. Once I got it showing in the shell, I forwarded this to a file.

Finally I used the mysql command line tool

to get access to the database, and then used the following command to import and run the SQL produced above.

And so far so good. I had been getting Server Errors on pages relating to the botched model before and at the moment they seem to be behaving correctly. Hopefully this may help you and not come back to haunt me.