old_contents: #Error: Site matching query does not exist.
  #Solved: http://stackoverflow.com/a/16068551/3445802
  
  $> ./manage.py shell
  >>> from django.contrib.sites.models import Site
  >>> site = Site()
  >>> site.domain = 'example.com'
  >>> site.name = 'example.com'
  >>> site.save()
  
new_contents: #Error: Site matching query does not exist.
  #Solved: http://stackoverflow.com/a/16068551/3445802
  
  #settinsg.py
  SITE_ID = 1
  INSTALLED_APPS = (
          ....
          'django.contrib.sites',
  )
  
  $> ./manage.py shell
  >>> from django.contrib.sites.models import Site
  >>> site = Site()
  >>> site.domain = 'example.com'
  >>> site.name = 'example.com'
  >>> site.save()
  
subject: Update sites.py