title: Invalid default value for user_id_id in django
body: <p>I am new to django and I am creating my models but I am having trouble when trying to add a foreign key to another model. here's my models: </p>
  
  <pre><code>from django.db import models
  
  class User(models.Model):
      user_id = models.CharField(max_length=10, primary_key=True)
      name = models.CharField(max_length=30)
      surname = models.CharField(max_length=30)
      role = models.CharField(max_length=10)
      address = models.CharField(max_length=50)
      email = models.EmailField(max_length=30)
      password = models.CharField(max_length=20)
      phone = models.IntegerField()
      GPA = models.FloatField(max_length=5)
      Gender = models.CharField(max_length=1)
  
      def __str__(self):
          return self.user_id
  
  
  class Login(models.Model):
      user_id = models.ForeignKey(User, on_delete=models.CASCADE,  default='00000')
      email = models.EmailField(max_length=30)
      password = models.CharField(max_length=20)
  
      def __str__(self):
          return self.email
  </code></pre>
  
  <p>When I type makemigrations I get this: </p>
  
  <pre><code>You are trying to change the nullable field 'user_id' on login to non-nullable without a default; we can't do that (the database needs something to populate existing rows).
  Please select a fix:
   1) Provide a one-off default now (will be set on all existing rows)
   2) Ignore for now, and let me handle existing rows with NULL myself (e.g. because you added a RunPython or RunSQL operation to handle NULL values in a previous data migration)
   3) Quit, and let me add a default in models.py
  Select an option:  3
  </code></pre>
  
  <p>So I added a default value but I am getting this error when I tried to migrate. I tried to change user_id from User to AutoField so I don't have to add any default value but it is still giving me this error. Plus I don't know why it says user_id_id at the end. Can anyone help me out with this? </p>
  
  <pre><code>    Running migrations:
        Rendering model states... DONE
        Applying login.0003_login_user_id...Traceback (most recent call last):
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 64, in execute
          return self.cursor.execute(sql, params)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\mysql\base.py", line 112, in execute
          return self.cursor.execute(query, args)
        File "C:\User\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 226, in execute
          self.errorhandler(self, exc, value)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\connections.py", line 42, in defaulterrorhandler
          raise errorvalue
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 223, in execute
          res = self._query(query)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 379, in _query
          rowcount = self._do_query(q)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 342, in _do_query
          db.query(q)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\connections.py", line 286, in query
          _mysql.connection.query(self, query)
      _mysql_exceptions.OperationalError: (1067, "Invalid default value for 'user_id_id'")
  
      The above exception was the direct cause of the following exception:
  
      Traceback (most recent call last):
        File "C:\Program Files (x86)\JetBrains\PyCharm 5.0.4\helpers\pycharm\django_manage.py", line 41, in &lt;module&gt;
          run_module(manage_file, None, '__main__', True)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\runpy.py", line 182, in run_module
          return _run_module_code(code, init_globals, run_name, mod_spec)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\runpy.py", line 96, in _run_module_code
          mod_name, mod_spec, pkg_name, script_name)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\runpy.py", line 85, in _run_code
          exec(code, run_globals)
        File "C:/Users/Desktop/Project/TSL/mysite\manage.py", line 10, in &lt;module&gt;
          execute_from_command_line(sys.argv)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\management\__init__.py", line 353, in execute_from_command_line
          utility.execute()
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\management\__init__.py", line 345, in execute
          self.fetch_command(subcommand).run_from_argv(self.argv)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\management\base.py", line 348, in run_from_argv
          self.execute(*args, **cmd_options)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\management\base.py", line 399, in execute
          output = self.handle(*args, **options)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\management\commands\migrate.py", line 200, in handle
          executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\migrations\executor.py", line 92, in migrate
          self._migrate_all_forwards(plan, full_plan, fake=fake, fake_initial=fake_initial)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\migrations\executor.py", line 121, in _migrate_all_forwards
          state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\migrations\executor.py", line 198, in apply_migration
          state = migration.apply(state, schema_editor)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\migrations\migration.py", line 123, in apply
          operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\migrations\operations\fields.py", line 62, in database_forwards
          field,
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\mysql\schema.py", line 50, in add_field
          super(DatabaseSchemaEditor, self).add_field(model, field)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\base\schema.py", line 396, in add_field
          self.execute(sql, params)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\base\schema.py", line 110, in execute
          cursor.execute(sql, params)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 79, in execute
          return super(CursorDebugWrapper, self).execute(sql, params)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 64, in execute
          return self.cursor.execute(sql, params)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\utils.py", line 95, in __exit__
          six.reraise(dj_exc_type, dj_exc_value, traceback)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\utils\six.py", line 685, in reraise
          raise value.with_traceback(tb)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 64, in execute
          return self.cursor.execute(sql, params)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\mysql\base.py", line 112, in execute
          return self.cursor.execute(query, args)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 226, in execute
          self.errorhandler(self, exc, value)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\connections.py", line 42, in defaulterrorhandler
          raise errorvalue
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 223, in execute
          res = self._query(query)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 379, in _query
          rowcount = self._do_query(q)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 342, in _do_query
          db.query(q)
        File "C:\Users\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\connections.py", line 286, in query
          _mysql.connection.query(self, query)
      django.db.utils.OperationalError: (1067, "Invalid default value for 'user_id_id'")
  
      Process finished with exit code 1
  </code></pre>