"ValueError: Lookup failed for model referenced by field app.Model.m2mfield: app.ThroghModel" in simple-self contained M2M setup

[Ramiro Morales]

In a simple ManyToManyField with intermediate model setup involving three models in the same application:

# app_b/models.py
from django.db import models

class Employee(models.Model):
    name = models.CharField(max_length=30)

class Employer(models.Model):
    name = models.CharField(max_length=30)
    employees = models.ManyToManyField(Employee, through='Employment')

class Employment(models.Model):
    employee = models.ForeignKey(Employee)
    employer = models.ForeignKey(Employer)
    recruiting_date = models.DateField()

I'm seeing the following traceback when running migrate after the initial migration has been created for the app:

~/dtest/dtest002$ rm -rf db.sqlite3 app_b/migrations

~/dtest/dtest002$ PYTHONPATH=~/django/upstream python ./manage.py makemigrations
No changes detected

ramiro@serverid:~/dtest/dtest002$ PYTHONPATH=~/django/upstream python ./manage.py makemigrations app_b
Migrations for 'app_b':
  0001_initial.py:
    - Create model Employee
    - Create model Employer
    - Create model Employment

~/dtest/dtest002$ PYTHONPATH=~/django/upstream python ./manage.py migrate
Operations to perform:
  Synchronize unmigrated apps: (none)
  Apply all migrations: admin, contenttypes, app_b, auth, sessions
Synchronizing apps without migrations:
  Creating tables...
  Installing custom SQL...
  Installing indexes...
Running migrations:
  Applying contenttypes.0001_initial... OK
  Applying auth.0001_initial... OK
  Applying admin.0001_initial... OK
  Applying app_b.0001_initial...Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/xxx/django/upstream/django/core/management/__init__.py", line 385, in execute_from_command_line
    utility.execute()
  File "/home/xxx/django/upstream/django/core/management/__init__.py", line 377, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/xxx/django/upstream/django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/home/xxx/django/upstream/django/core/management/base.py", line 337, in execute
    output = self.handle(*args, **options)
  File "/home/xxx/django/upstream/django/core/management/commands/migrate.py", line 146, in handle
    executor.migrate(targets, plan, fake=options.get("fake", False))
  File "/home/xxx/django/upstream/django/db/migrations/executor.py", line 62, in migrate
    self.apply_migration(migration, fake=fake)
  File "/home/xxx/django/upstream/django/db/migrations/executor.py", line 96, in apply_migration
    migration.apply(project_state, schema_editor)
  File "/home/xxx/django/upstream/django/db/migrations/migration.py", line 107, in apply
    operation.database_forwards(self.app_label, schema_editor, project_state, new_state)
  File "/home/xxx/django/upstream/django/db/migrations/operations/models.py", line 33, in database_forwards
    apps = to_state.render()
  File "/home/xxx/django/upstream/django/db/migrations/state.py", line 89, in render
    model=lookup_model,
ValueError: Lookup failed for model referenced by field app_b.Employer.employees: app_b.Employment

I'm selecting '1.7/beta-2' as Version but I'm running the latest stable/1.7.x code:

~/django/upstream$ git log  -1
commit 30d8b95139a1fa3070f3b112115e624ffcf8e555
Author: Tim Graham <timograham@gmail.com>
Date:   Thu Jun 19 11:02:53 2014 -0400

    [1.7.x] Added feature for implied null (needed for Firebird backend)

    Backport of 1b07781292 from master

Initially Employee didn't exist and the m2m relationship was against a custom User model.

After that trying to create a minimal setup that reproduces the issue, usage of the swappable user model feature was removed and the m2m field was re-pointed to standard auth.User, and finally to a local Employee model.