code: def get_prep_value(self, value):
          
          if isinstance(value, six.string_types):
              return value
          else:
              return ', '.join(map(lambda s: s.strip(), value))
docs: We need to accomodate queries where a single email,
          or list of email addresses is supplied as arguments. For example:
  
          - Email.objects.filter(to='mail@example.com')
          - Email.objects.filter(to=['one@example.com', 'two@example.com'])