title: What is the value of self._db by default in Django?
body: <p>Please see the following code:
      <code>user.save(using=self._db)</code>
  What is the default value of self._db for Django?
  Does this value default to what I've specified under default for database in my settings.py?</p>
  
  <p>I've found questions on Stack Overflow that say this value will provide a database type to Django, but if I've never set it explicitly, what is it by default?</p>