Django migrate AttributeError: 'str' object has no attribute '_meta'

Question

After doing some extensive changes in the DB schema, I ran makemigrations. It created migrations successfully. But then migrate failed with:

AttributeError: 'str' object has no attribute '_meta'

This is the code that I changed. I split up the Many to Many model for Hardskills from one through table to 2 different through table for users and jobs.

Initial

class Hardskills(models.Model):
    user = models.ManyToManyField(User, related_name="user_hs",through="HardskillsProfile")
    job = models.ManyToManyField(Job, related_name="job_hs",through="HardskillsProfile")
    hardskills = models.CharField(max_length=100, db_index=True)

    def __str__(self):
        return self.hardskills
    

class HardskillsProfile(models.Model):
    """Through Model for Many to Many relationship for user/jobs and hardskills"""
    user = models.ForeignKey(User, on_delete=models.CASCADE, related_name="user",null=True)
    job = models.ForeignKey(Job, on_delete=models.CASCADE, related_name="job",null=True)
    hardskills = models.ForeignKey(Hardskills, on_delete=models.CASCADE)

Final after changes

from attributes.models import Category
from django.contrib.auth.models import User
from content.models import Job
from django.db import models

class Hardskills(models.Model):
    user = models.ManyToManyField(User, related_name="user_hs",through="UserHardskillsProfile")
    job = models.ManyToManyField(Job, related_name="job_hs",through="JobHardskillsProfile")
    hardskills = models.CharField(max_length=100, db_index=True)

    def __str__(self):
        return self.hardskills
    

class UserHardskillsProfile(models.Model):
    """Through Model for Many to Many relationship for user and hardskills"""
    user = models.ForeignKey(User, on_delete=models.CASCADE, related_name="user",null=True)
    hardskills = models.ForeignKey(Hardskills, on_delete=models.CASCADE)

class JobHardskillsProfile(models.Model):
    """Through Model for Many to Many relationship for user/jobs and hardskills"""
    job = models.ForeignKey(Job, on_delete=models.CASCADE, related_name="job",null=True)
    hardskills = models.ForeignKey(Hardskills, on_delete=models.CASCADE)

Error Logs

Operations to perform:
  Synchronize unmigrated apps: messages, rest_framework, staticfiles
  Apply all migrations: accounts, admin, attributes, auth, authtoken, content, contenttypes, data, interactions, scoring, sessions
Synchronizing apps without migrations:
  Creating tables...
    Running deferred SQL...
Running migrations:
  Applying scoring.0017_auto_20201224_1023...Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    main()
  File "manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/core/management/__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/core/management/base.py", line 330, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/core/management/base.py", line 371, in execute
    output = self.handle(*args, **options)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/core/management/base.py", line 85, in wrapped
    res = handle_func(*args, **kwargs)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/core/management/commands/migrate.py", line 245, in handle
    fake_initial=fake_initial,
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/db/migrations/executor.py", line 227, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/db/migrations/migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/db/migrations/operations/fields.py", line 236, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/db/backends/sqlite3/schema.py", line 138, in alter_field
    super().alter_field(model, old_field, new_field, strict=strict)
  File "/home/yk09/miniconda3/lib/python3.7/site-packages/django/db/backends/base/schema.py", line 555, in alter_field
    old_field.remote_field.through._meta.auto_created and
AttributeError: 'str' object has no attribute '_meta'

Can anyone kindly advise? If my question is not clear, please do let me know as well :)

Answer 1

In the migration file scoring.0017_auto_20201224_1023... there will be list of operations. in that you can find a delete model migration.

move it to the end of migration list, like

From

    [
    ...    
    migrations.DeleteModel(
        name='HardskillsProfile',
    ),
    migrations.AlterField(...),
    ...
    ]

To

    [
    ...
    migrations.DeleteModel(
        name='HardskillsProfile',
    ],

then migrate.

Got this idea going through this ticket. which is supposably fixed, makemigrations is supposed to handle dangling reference which happens if a referenced model is deleted earlier in migration and referred later, so you can manually rearrange them.