Migration base/0074_drop_curated_thumbs.py broken

[etj]

The migration base/migrations/0074_drop_curated_thumbs.py may fail due to a race condition.

Migrations may fail with:

django4gn4_clean       |   Applying base.0073_resourcebase_thumbnail_path... OK
django4gn4_clean       |   Applying base.0074_drop_curated_thumbs...Error during updating resource: 'NoneType' object has no attribute 'objects'
django4gn4_clean       | Error during updating resource: 'NoneType' object has no attribute 'objects'
django4gn4_clean       | Error during updating resource: 'NoneType' object has no attribute 'objects'
[... repeated hundreds of times...]

By running the migration code by hand, it seems that the get_real_instance method is failing:

In [6]: resource = ResourceBase.objects.filter(id=resource_id).first()

In [7]: resource
Out[7]: <ResourceBase: Buffer Zone MaB Unesco Po Grande 2019 (Distretto Po)>

In [8]: resource.get_real_instance()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[8], line 1
----> 1 resource.get_real_instance()

File /usr/local/lib/python3.10/site-packages/polymorphic/models.py:173, in PolymorphicModel.get_real_instance(self)
    171 if real_model == self.__class__:
    172     return self
--> 173 return real_model.objects.db_manager(self._state.db).get(pk=self.pk)

AttributeError: 'NoneType' object has no attribute 'objects'

In [9]: resource.get_real_instance_class()

In [10]: type(resource)
Out[10]: geonode.base.models.ResourceBase

I noted that the layers migrations were not run, so the source code had a layers.Dataset class, where the DB still had the layers.Layer table.

Running by hand the migrations in the layers app and then running the base migrations, everything runs smoothly.