Custom migration operations are rewritten incorrectly

[Matthew Schinckel]

I've written a custom migration operation, and was in the process of writing a management command to write a migration file, using the MigrationWriter class.

It's almost all perfect, except that the MigrationWriter class assumes that the migration operation will be in django.db.migrations:

​https://github.com/django/django/blob/master/django/db/migrations/writer.py#L47

I suspect that the MigrationWriter will also be used for things like squashing migrations, so this problem may appear with custom migration operations then.

I suggest that it should inspect the migration operation, and if it's not in the django.db.migrations module, add the relevant module to the imports, and use the correct path for the operation.

[Matthew Schinckel]

So I've uploaded a patch that looks at the operation class, and decides if it should add an import (and use the fully qualified path for the operation).

[Baptiste Mispelon]

Hi,

This seems reasonable but the patch needs some tests.

Thanks.

[Matthew Schinckel]

Better patch, containing tests.

[Matthew Schinckel]

Better patch does contain tests, but I couldn't come up with a way to make the test fail when two classes have the same name, but a different path.

I needed to import the class using from ... import ... syntax, as otherwise there were issues related to namespacing and the term migrations, which I may try to work out.

[Matthew Schinckel]

I have a branch (almost) ready for a Pull Request.

​https://github.com/schinckel/django/tree/ticket_22788

[Matthew Schinckel]

The issue I have hit with this relates only to the django test framework.

Basically, the tests for this part live in the top-level 'migrations' package (when running the tests). Defining an operation within this package conflicts with the import in _every_ migrations file:

from django.db import migrations

This means I've had to import the custom operation directly (i.e., not namespaced).

Which then in turn means you cannot have two migration operations with the same name (that are actually different classes).

That is, I can import the foo.operations.Operation and bar.Operation in a normal situation, but can't actually write a passing test in the normal django test system.

I could mangle the names on import "from foo.operations import Operation as foo_operations_Operation", but this seems messy.

[Matthew Schinckel]

Perhaps code speaks louder than words.

The replacement code for adding the migration operation name to the migration file (and ensuring it is imported):

[OperationWriter.serialize()]

# See if this operation is in django.db.migrations. If it is,
# We can just use the fact we already have that imported,
# otherwise, we need to add an import for the operation class.
if getattr(migrations, name, None) == self.operation.class:

self.feed('migrations.%s(' % name)

else:

# This version works for the django tests, but fails if
# you have two identical named operations.
imports.add('from %s import %s' % (

self.operation.class.module, name

))
self.feed('%s(' % (name))

# This version works normally, including with same-named operations,
# but fails in the django tests, because
# self.operation.class.module will start with 'migrations'
imports.add('import %s' % self.operation.class.module)
self.feed('%s.%s(' % (

self.operation.class.module, name

))

[Andrew Godwin]

schinckel: I suspect you can get around this problem if you put this test in its own test app called custom_migration_operation ?

(We already have a few things in separate apps for a variety of reasons, they don't all need to be bundled into migrations)

Otherwise, the patch looks good. Sorry I overlooked this!

[anonymous]

I've submitted a ​pull request.

[Tim Graham <timograham@…>]

In bb39037fcbe07a4c4060764533b5c03a4018bf81:

Fixed #22788 -- Ensured custom migration operations can be written.

This inspects the migration operation, and if it is not in the
django.db.migrations module, it adds the relevant imports to the
migration writer and uses the correct class name.

[Tim Graham <timograham@…>]

In 2dba6ab767d8532df5f9ea2b58325ef823d3c6f8:

[1.7.x] Fixed #22788 -- Ensured custom migration operations can be written.

This inspects the migration operation, and if it is not in the
django.db.migrations module, it adds the relevant imports to the
migration writer and uses the correct class name.

Backport of bb39037fcb from master