title: Django: save() missing 1 required positional argument: 'self'
body: <p>I'm trying to override the save method and I think I write exactly what the document said, but get the error</p>
  
  <pre><code>from django.db import models
  
  class Post(models.Model):
      name = models.CharField(max_length = 20)
      lower_name = models.CharField(max_length = 20)
  
      def save(self, *args, **kwargs):
          if not self.lower_name:
              self.lower_name = self.name.lower()
          super().save(*args, **kwargs)
  </code></pre>
  
  <p>here is the traceback when I call python manage.py migrate</p>
  
  <pre><code>(project_site) λ python manage.py migrate
  Operations to perform:
    Apply all migrations: admin, auth, contenttypes, sessions, works
  Running migrations:
    Applying works.0018_post_url_name...Traceback (most recent call last):
    File "manage.py", line 15, in &lt;module&gt;
      execute_from_command_line(sys.argv)
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\core\management\__init__.py", line 381, in execute_from_command_line
      utility.execute()
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\core\management\__init__.py", line 375, in execute
      self.fetch_command(subcommand).run_from_argv(self.argv)
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\core\management\base.py", line 323, in run_from_argv
      self.execute(*args, **cmd_options)
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\core\management\base.py", line 364, in execute
      output = self.handle(*args, **options)
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\core\management\base.py", line 83, in wrapped
      res = handle_func(*args, **kwargs)
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\core\management\commands\migrate.py", line 234, in handle
      fake_initial=fake_initial,
    File "D:\Code\Python\project_site\.venv\lib\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 "D:\Code\Python\project_site\.venv\lib\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 "D:\Code\Python\project_site\.venv\lib\site-packages\django\db\migrations\executor.py", line 245, in apply_migration
      state = migration.apply(state, schema_editor)
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\db\migrations\migration.py", line 124, in apply
      operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\db\migrations\operations\fields.py", line 112, in database_forwards
      field,
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\db\backends\sqlite3\schema.py", line 327, in add_field
  
      self._remake_table(model, create_field=field)
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\db\backends\sqlite3\schema.py", line 188, in _remake_table
      self.effective_default(create_field)
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\db\backends\base\schema.py", line 233, in effective_default
      return field.get_db_prep_save(self._effective_default(field), self.connection)
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\db\backends\base\schema.py", line 212, in _effective_default
      default = field.get_default()
    File "D:\Code\Python\project_site\.venv\lib\site-packages\django\db\models\fields\__init__.py", line 797, in get_default
      return self._get_default()
  TypeError: save() missing 1 required positional argument: 'self'
  </code></pre>
  
  <blockquote>
    <p>python version: 3.6.8 django version: 2.2.4</p>
  </blockquote>