One of Django’s most powerful features is the ORM, which includes a robust migration framework. One of Django’s most misunderstood features is Django migrations because it just works 99% of the time.
Even when working solo, Django migrations are highly reliable, working 99.9% of the time and offering better uptime than most web services you may have used last week.
The most common stumbling block for developers of all skill levels is rolling back a Django migration and prepping a pull request for review.
I’m not picky about pull requests or git commit history because I default to using the “Squash and merge” feature to turn all pull request commits into one merge commit. The merge commit tells me when, what, and why something changed if I need extra context.
I am pickier about seeing >2 database migrations for any app unless a data migration is involved. It’s common to see 4 to 20 migrations when someone works on a database feature for a week. Most of the changes tend to be fiddly, where someone adds a field, renames the field, renames it again, and then starts using it, which prompts another null=True change followed by a blank=True migration.
For small databases, none of this matters.
For a database with 10s or 100s of millions of records, these small changes can cause minutes of downtime per migration, which amounts to a throwaway change. While there are ways to mitigate most migration downtime situations, that’s different from my point today.
I’m also guilty of being fiddly with my Django model changes because I know I can delete and refactor them before requesting approval. The process I use is probably worth sharing because once every new client comes up.
Let’s assume I am working on Django News Jobs, and I am looking over my pull request one last time before I ask someone to review it. That’s when I noticed four migrations that could quickly be rebuilt into one, starting with my 0020* migration in my jobs app.
The rough steps that I would do are:
# step 1: see the state of our migrations
$ python -m manage showmigrations jobs
jobs
[X] 0001_initial
...
[X] 0019_alter_iowa_versus_unconn
[X] 0020_alter_something_i_should_delete
[X] 0021_alter_uconn_didnt_foul
[X] 0022_alter_nevermind_uconn_cant_rebound
[X] 0023_alter_iowa_beats_uconn
[X] 0024_alter_south_carolina_sunday_by_four
# step 2: rollback migrations to our last "good" state
$ python -m manage migrate jobs 0019
# step 3: delete our new migrations
$ rm jobs/migrations/002*
# step 4: rebuild migrations
python -m manage makemigrations jobs
# step 5: profit
python -m manage migrate jobs
95% of the time, this is all I ever need to do.
Occasionally, I check out another branch with conflicting migrations, and I’ll get my local database in a weird state.
In those cases, check out the --fake (“Mark migrations as run without actually running them.") and --prune (“Delete nonexistent migrations from the django_migrations table.") options. The fake and prune operations saved me several times when my django_migrations table was out of sync, and I knew that SQL tables were already altered.
What not squashmigrations?
Excellent question. Squashing migrations is wonderful if you care about keeping every or most of the operations each migration is doing. Most of the time, I do not, so I overlook it.
