title: add() for ManyToManyField which specifies an intermediary model
body: <p>There are many questions about this Exception:</p>
  
  <pre><code>AttributeError: Cannot set values on a ManyToManyField which specifies 
  an intermediary model.
  Use myapp.MyThroughModel's Manager instead.
  </code></pre>
  
  <p>The <a href="https://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-many-to-many-relationships" rel="noreferrer">docs</a></p>
  
  <blockquote>
    <p>You can’t just create a relationship between a Person and a Group -
    you need to specify all the detail for the relationship required by
    the Membership model. The simple add, create and assignment calls
    don’t provide a way to specify this extra detail. As a result, they
    are disabled for many-to-many relationships that use an intermediate
    model.</p>
  </blockquote>
  
  <p>But all my attributes/columns of MyThroughModel are optional. The manager could create them.</p>
  
  <p>How can I write a custom manager to automatically create MyThroughModel instances?</p>
  
  <p>I need a solution at ORM-level, not Form-level.</p>