title: Django migration failed with un-migrating m2m through field change
body: <p>I am trying to migrate one of my models back to remove this change but I am getting the traceback below.</p>
  
  <pre><code>  File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 354, in execute_from_command_line
  utility.execute()
    File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 346, in execute
      self.fetch_command(subcommand).run_from_argv(self.argv)
    File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 394, in run_from_argv
      self.execute(*args, **cmd_options)
    File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 445, in execute
      output = self.handle(*args, **options)
    File "/usr/local/lib/python2.7/dist-packages/django/core/management/commands/migrate.py", line 222, in handle
      executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
    File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/executor.py", line 112, in migrate
      self.unapply_migration(states[migration], migration, fake=fake)
    File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/executor.py", line 168, in unapply_migration
      state = migration.unapply(state, schema_editor)
    File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/migration.py", line 162, in unapply
      operation.database_backwards(self.app_label, schema_editor, from_state, to_state)
    File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/operations/fields.py", line 206, in database_backwards
      self.database_forwards(app_label, schema_editor, from_state, to_state)
    File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/operations/fields.py", line 201, in database_forwards
      schema_editor.alter_field(from_model, from_field, to_field)
    File "/usr/local/lib/python2.7/dist-packages/django/db/backends/base/schema.py", line 473, in alter_field
      not new_field.rel.through._meta.auto_created):
  AttributeError: 'unicode' object has no attribute '_meta'
  </code></pre>
  
  <p>Here is the migration</p>
  
  <pre><code>class Migration(migrations.Migration):
  
      dependencies = [
          ('app', '0004_auto_20151130_2345'),
      ]
  
      operations = [
          migrations.RenameModel('OldModelName', 'NewModelName'),
          migrations.AlterField(
              model_name='adifferentmodel',
              name='them2mfieldname',
              field=models.ManyToManyField(to='app.TheSecondModel', through='app.NewModelName', blank=True),
          ),
  
      ]
  </code></pre>