Pulp

If you create a relation (foreign key, onetoone, manytomany) in a plugin to any model redefined in plugin API (pulpcore.plugin.models), the following error occurs:

 $ django-admin makemigrations
    Traceback (most recent call last):
      File "/usr/local/lib/pulp/bin/django-admin", line 11, in <module>
        sys.exit(execute_from_command_line())
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
        utility.execute()
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/management/__init__.py", line 375, in execute
        self.fetch_command(subcommand).run_from_argv(self.argv)
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/management/base.py", line 323, in run_from_argv
        self.execute(*args, **cmd_options)
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/management/base.py", line 361, in execute
        self.check()
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/management/base.py", line 390, in check
        include_deployment_checks=include_deployment_checks,
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/management/base.py", line 377, in _run_checks
        return checks.run_checks(**kwargs)
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/checks/registry.py", line 72, in run_checks
        new_errors = check(app_configs=app_configs)
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/checks/model_checks.py", line 168, in check_lazy_references
        return _check_lazy_references(apps)
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/checks/model_checks.py", line 163, in _check_lazy_references
        )), key=lambda error: error.msg)
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/checks/model_checks.py", line 162, in <genexpr>
        for func in apps._pending_operations[model_key]
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/checks/model_checks.py", line 157, in build_error
        return error_fn(model_key, func, args, keywords) if error_fn else None
      File "/usr/local/lib/pulp/lib64/python3.6/site-packages/django/core/checks/model_checks.py", line 107, in field_error
        'model': '.'.join(model_key),
    TypeError: sequence item 0: expected str instance, NoneType found

The reason is that those models are [re]defined in pulpcore-plugin, and have no app_label attached to them. This causes django lazy reference checks to fail at the migration generation time.

The workaround is to import directly from the pulpcore.

from pulpcore.plugin.models import Remote                                                                                                            
print(Remote._meta.app_label)  # None

from pulpcore.app.models import Remote                                                                                                               
print(Remote._meta.app_label)  # 'core'

Affected models (basically all models in pulpcore-plugin):
- Content
- ContentGuard
- Remote
- Publisher