title: Specify what database to use when adding relation
body: <p>I know I can specify a database to use with "using" keyword like this:</p>
  
  <pre><code>zoo = Zoo.objects.using(db_id).get(pk=123)
  </code></pre>
  
  <p>But how can I specify database to use when I am adding a relation?</p>
  
  <pre><code>animal = Animal(name="Lion")
  zoo.animals.add(animal)
  </code></pre>