Name clash of merge migrations after squashing

[Tim Graham]

For clarity, could you give the specific steps that led to the clash?

[Raphael Gaschignard]

I just ran into a similar problem, going to detail what happened:

• Over the course of development, do a merge migration. In our case, it was the 21st migration so got named 0021_merge.py

• Squash our migrations, deleting 0021_merge.py

• Continue development, and through sheer luck, our next 2st migration is also a merge commit. It gets named 0021_merge.py

• When running migrations, because the original migration was run, this new 0021_merge.py commit is not run

In my case, this was the dependencies of 0021_merge.py (the new one):

    dependencies = [
        ('document', '0020_auto_20160413_1834'),
        ('document', '0019_auto_20160414_1150'),
    ]

Because the merge commit wasn't run, some people only had the 0020 mainline migrations migrated, some people only had the 0019 branch migrations run, and some people (who hadn't run migrate in a while) had the following happen:

• migrate detects 0021_merge.py as the leaf node
• there was record of the (old) 0021_merge.py migration
• the system was like "oh, guess you're up to date!" and then doesn't run either branch of the merge commit

Like the OP, I believe the simplest solution is to timestamp the merge migrations.

[Raphael Gaschignard]

Did a bit more digging into this, I believe that the fix is changing [(​https://github.com/django/django/blob/master/django/core/management/commands/makemigrations.py#L286)], from :

new_migration = subclass("%04i_merge" % (biggest_number + 1), app_label) 

to :

timestamp = datetime.datetime.now().strftime("%Y%m%d_%H%M")
new_migration = subclass("%04i_merge_%s" % (biggest_number + 1, timestamp), app_label) 

I'm not quite sure how to write tests for this, though. You could write some models, generate migrations, squash, generate new ones and then confirm that everything got run? But I feel like the current squash migration tests also cover this, and that there isn't really much in term of name conflict tests.

[Daniel Izquierdo]

The docs in ​https://github.com/django/django/blob/2cd2d188516475ddf256e6267cd82c495fb5c430/django/db/migrations/autodetector.py#L1129 say that names are not guaranteed to be unique and the timestamp is just a best effort to avoid conflicts. So I think rtpg's suggested fix works well for merge migrations, and a test to ensure names don't conflict is not necessary.