Model keys are too long on MySQL with utf8mb4 collation

[cassidylaidlaw]

Steps to reproduce:

• Set up Django project with push_notifications app installed
• Connect to MySQL database
• Run ALTER DATABASE database_name CHARACTER SET = utf8mb4 COLLATE = utf8mb4_unicode_ci;
• Try to run manage.py migrate

It produces the following output:

  Apply all migrations: push_notifications
Running migrations:
  Applying push_notifications.0001_initial... OK
  Applying push_notifications.0002_auto_20160106_0850...Traceback (most recent call last):
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/backends/mysql/base.py", line 71, in execute
    return self.cursor.execute(query, args)
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/cursors.py", line 250, in execute
    self.errorhandler(self, exc, value)
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/connections.py", line 50, in defaulterrorhandler
    raise errorvalue
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/cursors.py", line 247, in execute
    res = self._query(query)
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/cursors.py", line 412, in _query
    rowcount = self._do_query(q)
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/cursors.py", line 375, in _do_query
    db.query(q)
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/connections.py", line 276, in query
    _mysql.connection.query(self, query)
_mysql_exceptions.OperationalError: (1071, 'Specified key was too long; max key length is 767 bytes')

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

Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/core/management/base.py", line 316, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/core/management/base.py", line 353, in execute
    output = self.handle(*args, **options)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/core/management/base.py", line 83, in wrapped
    res = handle_func(*args, **kwargs)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 203, in handle
    fake_initial=fake_initial,
  File "/opt/python/run/venv/local/lib/python3.6/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 "/opt/python/run/venv/local/lib/python3.6/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 "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/migrations/migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/migrations/operations/fields.py", line 216, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 523, in alter_field
    old_db_params, new_db_params, strict)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 663, in _alter_field
    params,
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 133, in execute
    cursor.execute(sql, params)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 68, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 77, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/utils.py", line 89, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
  File "/opt/python/run/venv/local/lib/python3.6/site-packages/django/db/backends/mysql/base.py", line 71, in execute
    return self.cursor.execute(query, args)
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/cursors.py", line 250, in execute
    self.errorhandler(self, exc, value)
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/connections.py", line 50, in defaulterrorhandler
    raise errorvalue
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/cursors.py", line 247, in execute
    res = self._query(query)
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/cursors.py", line 412, in _query
    rowcount = self._do_query(q)
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/cursors.py", line 375, in _do_query
    db.query(q)
  File "/opt/python/run/venv/local/lib64/python3.6/site-packages/MySQLdb/connections.py", line 276, in query
    _mysql.connection.query(self, query)
django.db.utils.OperationalError: (1071, 'Specified key was too long; max key length is 767 bytes')

This seems to be because some unique fields are longer than 767 bytes when accounting for the 4-byes-per-character in the utf8mb4 encoding.