Avoiding clashing Django migrations

Managing South migrations on a multi-developer Django project can be painful. Developers working on separate branches will often create migrations for the same app with the same migration number1. When merged into master, these clashing migrations can cause deployment hiccups as South will complain if migrations are applied out of order.

There are various techniques available for dealing with this2, but what we do at Yoyo is test for such clashes as part of our Travis continuous integration.

This is done by calling a makefile target from .travis.yml:

# .travis.yml

language: python

    - 2.7

    - make virtualenv

    - make test
    - make migration_test

where the migration_test target is:

# makefile

MIGRATION_CLASHES=$(shell find . -type f -name "*.py" | grep -o ".*/migrations/[0-9]\+" | sort | uniq -c | awk '$$1 > 1 {print $$0}')
    [ -n $(MIGRATION_CLASHES) ] && exit 1 || true

Here the $(shell ...) call extracts the app name and migration number from all migration files then uses awk to look for clashes. If any are found, the Travis build will fail and the console output should reveal which apps have clashes.

This works best if you only allow fast-forward commits into master (something we do at Yoyo). Doing this forces you to merge master back into your pull request branch and allows Travis to catch migration clashes before it is merged. Then any conflicts can be resolved by renumbering or recreating any migrations not yet merged to master.

  1. I’m only talking about Django versions less than 1.7 - I’m not sure if this is still an issue in more modern Django versions.

  2. As noted by the South docs, you can run the migrations with the --merge option although this generally means a manual intervention in your deployment process which isn’t ideal.


Tagged with: django, testing
Filed in: tips
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