Django: 1215, 'Cannot add foreign key constraint' on model with only one field

Question

I have a Salad model:

class Salad(models.Model):
    some_field = models.ForeignKey(Profile, on_delete=models.CASCADE, null=True)

No matter how I call the class, the single field, what models I put instead of Profile, or whichever on_delete option I choose. I systematically get the error:

django.db.utils.IntegrityError: (1215, 'Cannot add foreign key constraint')

The error log:

Traceback (most recent call last):
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 85, in _execute
  Applying playerdata.0053_auto_20180724_1943...    return self.cursor.execute(sql, params)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\mysql\base.py", line 71, in execute
    return self.cursor.execute(query, args)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 250, in execute
    self.errorhandler(self, exc, value)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\connections.py", line 50, in defaulterrorhandler
    raise errorvalue
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 247, in execute
    res = self._query(query)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 411, in _query
    rowcount = self._do_query(q)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 374, in _do_query
    db.query(q)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\connections.py", line 292, in query
    _mysql.connection.query(self, query)
_mysql_exceptions.IntegrityError: (1215, 'Cannot add foreign key constraint')

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "C:\Program Files\JetBrains\PyCharm 2017.3.2\helpers\pycharm\django_manage.py", line 52, in <module>
    run_command()
  File "C:\Program Files\JetBrains\PyCharm 2017.3.2\helpers\pycharm\django_manage.py", line 46, in run_command
    run_module(manage_file, None, '__main__', True)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\runpy.py", line 205, in run_module
    return _run_module_code(code, init_globals, run_name, mod_spec)
  File "C:\Users\Div-o\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\Div-o\AppData\Local\Programs\Python\Python35-32\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "E:/Python/DjangoProjects/rolegate\manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\management\__init__.py", line 371, in execute_from_command_line
    utility.execute()
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\management\__init__.py", line 365, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\management\base.py", line 288, in run_from_argv
    self.execute(*args, **cmd_options)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\management\base.py", line 335, in execute
    output = self.handle(*args, **options)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\core\management\commands\migrate.py", line 200, in handle
    fake_initial=fake_initial,
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\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 "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\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 "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\migrations\executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\base\schema.py", line 90, in __exit__
    self.execute(sql)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\base\schema.py", line 117, in execute
    cursor.execute(sql, params)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 100, in execute
    return super().execute(sql, params)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 68, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 77, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\utils.py", line 89, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django\db\backends\mysql\base.py", line 71, in execute
    return self.cursor.execute(query, args)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 250, in execute
    self.errorhandler(self, exc, value)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\connections.py", line 50, in defaulterrorhandler
    raise errorvalue
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 247, in execute
    res = self._query(query)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 411, in _query
    rowcount = self._do_query(q)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\cursors.py", line 374, in _do_query
    db.query(q)
  File "C:\Users\Div-o\AppData\Local\Programs\Python\Python35-32\lib\site-packages\MySQLdb\connections.py", line 292, in query
    _mysql.connection.query(self, query)
django.db.utils.IntegrityError: (1215, 'Cannot add foreign key constraint')

Answer 1

Normally it is caused by the fact that your two related tables are not using InnoDB and at least one of them uses MyISAM engine instead.

The problem is Foreign keys are not supported on MyISAM.

Here is a related Django ticket #26777.

Since this is entirely related to MYSQL and not a Django specific, I will link this thread that could potentially help you,

Converting Tables from MyISAM to InnoDB

FUN FACT

I'm not the one that took care of this Django ticket even if the names are similar. @SimonCharette is.