title: Django: update field of ManyToMany instance when it's set
body: <p>Is it possible to do something when setting ManyToMany relations with <code>.set()</code>?</p>
  <p>Let's say, we have:</p>
  <pre><code>class ModelA():
    b_models = models.ManyToManyField(ModelB, related_name=&quot;as&quot;, through=&quot;ModelMN&quot;)
  
  class ModelMN():
    model_a = models.ForeignKey(ModelA)
    model_b = models.ForeignKey(ModelB)
  </code></pre>
  <p>And when we have an instance of <strong>Model A</strong> and list of <strong>Model B</strong> instances:</p>
  <pre><code>a = ModelA()
  a.set([b1, b2, b3])
  </code></pre>
  <p>When calling <code>.set()</code>, an instace of <strong>ModelMN</strong> is created for every a-b relationship. Is it possible to modify a field of that instance (every ModelMN instance), everytime it is created?</p>
  <p>Overriding <code>.save()</code> method on <code>ModelMN</code> seems not to be working.</p>