title: Error after creating a foreign key: django.db.utils.IntegrityError: MyApp_job__new.level_id may not be NULL
body: <p>I've created a new attribute <code>level</code> in my model. I've forgot to put there <code>null=True</code> before migrations. When the cmd asks whether I want to put a default value, I've typed <code>None</code>. Now, I can't do <code>migrate</code>. So I tried to remove <code>level</code> column and do new <code>makemigrations</code> but still the same problem happens when I try to call <code>migrate</code>.</p>
  
  <pre><code>django.db.utils.IntegrityError: MyApp__job__new.level_id may not be NULL
  </code></pre>
  
  <p>Is there a way to get back to the state when there was no <code>level</code> attribute?</p>
  
  <pre><code>makemigrations:
  
  Migrations for 'SolutionsForLanguagesApp':
    0019_remove_job_level.py:
      - Remove field level from job
  </code></pre>
  
  <p>migrate:</p>
  
  <pre><code>(venv) C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2&gt;pyth
  on manage.py migrate
  Operations to perform:
    Synchronize unmigrated apps: essay, true_false, super_inlines, staticfiles, mu
  ltichoice, messages, quiz, django_tables2, crispy_forms
    Apply all migrations: admin, contenttypes, SolutionsForLanguagesApp, auth, ses
  sions
  Synchronizing apps without migrations:
    Creating tables...
      Running deferred SQL...
    Installing custom SQL...
  Running migrations:
    Rendering model states... DONE
    Applying SolutionsForLanguagesApp.0013_auto_20160413_1615...Traceback (most re
  cent call last):
    File "manage.py", line 10, in &lt;module&gt;
      execute_from_command_line(sys.argv)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\core\management\__init__.py", line 354, in execute_fr
  om_command_line
      utility.execute()
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\core\management\__init__.py", line 346, in execute
      self.fetch_command(subcommand).run_from_argv(self.argv)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\core\management\base.py", line 394, in run_from_argv
      self.execute(*args, **cmd_options)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\core\management\base.py", line 445, in execute
      output = self.handle(*args, **options)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\core\management\commands\migrate.py", line 222, in ha
  ndle
      executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\migrations\executor.py", line 110, in migrate
      self.apply_migration(states[migration], migration, fake=fake, fake_initial=f
  ake_initial)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\migrations\executor.py", line 148, in apply_migrat
  ion
      state = migration.apply(state, schema_editor)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\migrations\migration.py", line 115, in apply
      operation.database_forwards(self.app_label, schema_editor, old_state, projec
  t_state)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\migrations\operations\fields.py", line 62, in data
  base_forwards
      field,
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\backends\sqlite3\schema.py", line 179, in add_fiel
  d
      self._remake_table(model, create_fields=[field])
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\backends\sqlite3\schema.py", line 147, in _remake_
  table
      self.quote_name(model._meta.db_table),
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\backends\base\schema.py", line 111, in execute
      cursor.execute(sql, params)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\backends\utils.py", line 79, in execute
      return super(CursorDebugWrapper, self).execute(sql, params)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\backends\utils.py", line 64, in execute
      return self.cursor.execute(sql, params)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\utils.py", line 98, in __exit__
      six.reraise(dj_exc_type, dj_exc_value, traceback)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\backends\utils.py", line 64, in execute
      return self.cursor.execute(sql, params)
    File "C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2\ven
  v\lib\site-packages\django\db\backends\sqlite3\base.py", line 318, in execute
      return Database.Cursor.execute(self, query, params)
  django.db.utils.IntegrityError: SolutionsForLanguagesApp_job__new.level_id may n
  ot be NULL
  </code></pre>
  
  <p>SHOWMIGRATIONS:</p>
  
  <pre><code>(venv) C:\Users\Milano\PycharmProjects\FutileStudio\SolutionsForLanguages_2&gt;pyth
  on manage.py showmigrations SolutionsForLanguagesApp
  SolutionsForLanguagesApp
   [X] 0001_initial
   [X] 0002_auto_20160401_1234
   [X] 0003_auto_20160402_0105
   [X] 0004_auto_20160402_0107
   [X] 0005_job_is_done
   [X] 0006_auto_20160408_1506
   [X] 0007_auto_20160411_1834
   [X] 0008_auto_20160413_1413
   [X] 0009_auto_20160413_1446
   [X] 0010_auto_20160413_1543
   [X] 0011_auto_20160413_1559
   [X] 0012_auto_20160413_1604
   [ ] 0013_auto_20160413_1615
   [ ] 0014_auto_20160413_1616
   [ ] 0015_auto_20160413_1616
   [ ] 0016_remove_job_level
   [ ] 0017_job_level
   [ ] 0018_auto_20160413_1627
   [ ] 0019_remove_job_level
  </code></pre>
  
  <p>MIGRATION 0013:</p>
  
  <pre><code># -*- coding: utf-8 -*-
  from __future__ import unicode_literals
  
  from django.db import migrations, models
  from django.conf import settings
  
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('SolutionsForLanguagesApp', '0012_auto_20160413_1604'),
      ]
  
      operations = [
          migrations.AddField(
              model_name='job',
              name='level',
              field=models.ForeignKey(default=None, blank=True, to='SolutionsForLanguagesApp.Level'),
              preserve_default=False,
          ),
          migrations.AlterField(
              model_name='job',
              name='customer',
              field=models.ForeignKey(related_name='orders', to=settings.AUTH_USER_MODEL, help_text='Z\xe1kazn\xedk'),
          ),
          migrations.AlterField(
              model_name='job',
              name='file',
              field=models.FileField(help_text='Upload file', null=True, upload_to=b'', blank=True),
          ),
          migrations.AlterField(
              model_name='job',
              name='price',
              field=models.FloatField(help_text='Cena', null=True, blank=True),
          ),
          migrations.AlterField(
              model_name='job',
              name='translator',
              field=models.ForeignKey(related_name='jobs', blank=True, to=settings.AUTH_USER_MODEL, help_text='Prekladate\u013e', null=True),
          ),
      ]
  </code></pre>