In the event of a failure, make migrations output the operation that failed

[Brandon Moser]

After creating a migration, via makemigrations, I attempted to run sqlmigrate, but received the following error. I have 3 migrations using the object referenced. It would be helpful to add the exact migration operation in the traceback. I think the migration traceback should, also, show the model/table and column that is part of the error.

Traceback (most recent call last):
  File "manage.py", line 11, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 377, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/commands/sqlmigrate.py", line 30, in execute
    return super(Command, self).execute(*args, **options)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 338, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/commands/sqlmigrate.py", line 61, in handle
    sql_statements = executor.collect_sql(plan)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/executor.py", line 82, in collect_sql
    migration.apply(project_state, schema_editor, collect_sql=True)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/migration.py", line 108, in apply
    operation.database_forwards(self.app_label, schema_editor, project_state, new_state)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/operations/fields.py", line 139, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/schema.py", line 460, in alter_field
    return self._alter_many_to_many(model, old_field, new_field, strict)
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/schema.py", line 764, in _alter_many_to_many
    new_field.rel.through._meta.get_field_by_name(new_field.m2m_reverse_field_name())[0],
  File "/usr/local/lib/python2.7/dist-packages/django/utils/functional.py", line 17, in _curried
    return _curried_func(*(args + moreargs), **dict(kwargs, **morekwargs))
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 2224, in _get_m2m_reverse_attr
    return getattr(self, cache_attr)
AttributeError: 'VersionedManyToManyField' object has no attribute '_m2m_reverse_name_cache'

Note: The issue with the VersionedManyToManyField has a reported issue with the CleanerVersion team and can be found ​(here).

[Tim Graham]

I am not sure if such a generic exception reporting mechanism is feasible and/or practical to implement. Did you have any ideas about that?

I typically use pdb inside Django's source code to get the needed information.

[Brandon Moser]

Replying to timgraham:

I am not sure if such a generic exception reporting mechanism is feasible and/or practical to implement. Did you have any ideas about that?

I typically use pdb inside Django's source code to get the needed information.

As for exact implementation, I'm not sure. I'm pretty new to Python/Django and haven't used pdb yet (I'll be digging into that next). I think the frustration that I've seen in other tickets is that this is a failure following a generated migration. If I had created a migration manually, then I'm ok with obscure errors, since I should have a better understanding of what I'm doing.

Here's what I would have liked to have seen in the error:

AttributeError: 'VersionedManyToManyField' object has no attribute '_m2m_reverse_name_cache'. Seen on line 257 in app_name/0003_auto_20150721_1018.py.

OR

AttributeError: 'VersionedManyToManyField' object has no attribute '_m2m_reverse_name_cache' on `model_name.field`

Unfortunately for me, I've been fighting migrations for the last couple days (due to some complicated changes) and have had to deal with errors like this.

[Shai Berger]

I agree that migration stacktraces are missing references to the migrations which caused them. Perhaps it can be helpful to add debugging hints for migrations, e.g. "increase verbosity so you can tell which migration caused the problem".

[Tim Graham]

Yes, I have used that technique. A section in docs/topics/migrations.txt with tips on debugging migrations sounds good to me.

[Markus Holtermann]

I've a patch laying around to output the exact operation and migration that failed. Only struggling with the connection resets when a migration failed, but that shouldn't be a problem to fix. Will push and link when I've decent WiFi to rebase onto current master.

[Markus Holtermann]

Here you go: ​https://github.com/django/django/compare/master...MarkusH:migration-traceback

Note the raise NotImplementedError() in test_debug_migrate_exception(). I have no idea to fix the issues that occur when you drop that line.