Migration changing ManyToManyField target to 'self' doesn't work correctly

[MSleepyPanda]

Steps to reproduce:
Create Models:

class Bar(models.Model):
    pass

class Foo(models.Model):
    bar = models.ManyToManyField('Bar', blank=True)

Migrate:

./manage.py makemigrations app
./manage.py migrate

Change type of the ManyToManyField to Foo:

class Bar(models.Model):
    pass

class Foo(models.Model):
    bar = models.ManyToManyField('Foo', blank=True)

Migrate (see above)

In the admin page, navigate to "add Foo", click save

You should see an OperationalError, "no such column: app_foo_bar.from_foo_id"

[Tim Graham]

I believe it works correctly as long as the new target model isn't 'self'.

[SShayashi]

Can I check out what you want? You can use 'self' instead of 'Foo' like this :

class Foo(models.Model):
    bar = models.ManyToManyField('self', blank=True)

You meant that we should use 'Foo' rather than 'self', right?

[MSleepyPanda]

Replying to SShayashi:

Can I check out what you want? You can use 'self' instead of 'Foo' like this :

class Foo(models.Model):
    bar = models.ManyToManyField('self', blank=True)

You meant that we should use 'Foo' rather than 'self', right?

Exactly, though i wasn't aware that self would work in that context. I think i like naming the type directly more, since self could be easily confused with the self argument of class methods.

[Bhuvnesh]

While altering a many to many field , a new table is created (which is later renamed) and data is copied from old table to new table and then old table is deleted. This issue caused while making this new table ​here, we are only altering the m2m_reverse_fields ignoring the m2m_fields that points to our model. But both m2m_reverse_fieldsand m2m_fields needs to be changed.
Hope i'm proceeding into the right direction.

[Bhuvnesh]

It is also failing for the case if process is reversed, like when we migrate passing self/Foo to the m2m field and then change it to Bar.(due to the same reason that we are not altering m2m_fields)

I tried to alter self pointing field and with the changes below its working as expected for the above issue and passing all the tests as well.

django/db/backends/sqlite3/schema.py

@@ -174 +174 @@
             super().alter_field(model, old_field, new_field, strict=strict)
 
     def _remake_table(
-        self, model, create_field=None, delete_field=None, alter_field=None
+        self, model, create_field=None, delete_field=None, alter_field=None, alter_self_field=None
     ):
         """
         Shortcut to transform a model from old_model into new_model
@@ -236 +236 @@
                 mapping[create_field.column] = self.prepare_default(
                     self.effective_default(create_field),
                 )
+
+        # Alter field pointing to the model itself.
+        if alter_self_field:
+            old_self_field, new_self_field = alter_self_field
+            body.pop(old_self_field.name, None)
+            mapping.pop(old_self_field.column, None)
+            body[new_self_field.name] = new_self_field
+            mapping[new_self_field.column] = self.quote_name(old_self_field.column)
+            rename_mapping[old_self_field.name] = new_self_field.name
+
         # Add in any altered fields
         if alter_field:
             old_field, new_field = alter_field
@@ -507 +517 @@
                         new_field.m2m_reverse_field_name()
                     ),
                 ),
+                alter_self_field=(
+                    old_field.remote_field.through._meta.get_field(
+                        old_field.m2m_field_name()
+                    ),
+                    new_field.remote_field.through._meta.get_field(
+                        new_field.m2m_field_name()
+                    ),
+                ),
             )
             return

[Carlton Gibson]

Hi Bhuvnesh

I tried to alter self pointing field and with the changes below its working as expected for the above issue and passing all the tests as well.

If the tests are all passing it's worth opening a PR to make review easier. (Make sure to add a new test for the issue here too.)

Thanks.

[Bhuvnesh]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 81b1c16:

Fixed #28987 -- Fixed altering ManyToManyField when changing to self-referential.