Django, after upgrade: MySQL server has gone away

The reason of such behavior is persistent connect to database, which was introduced in Django 1.6.

To prevent connection timeout error you should set CONN_MAX_AGE in settings.py to value which is less than wait_timeout in MySQL config (my.cnf). In that case Django detects that connection need to be reopen earlier than MySQL throws it. Default value for MySQL 5.7 is 28800 seconds.

settings.py:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.mysql',
        'CONN_MAX_AGE': 3600,
        <other params here>
    }
}

Documentation: https://docs.djangoproject.com/en/1.7/ref/settings/#conn-max-age

my.cnf:

wait_timeout = 28800

Documentation: https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_wait_timeout