title: Get current user in Model Serializer
body: <p>Is it possible to get the current user in a model serializer? I'd like to do so without having to branch away from generics, as it's an otherwise simple task that must be done.</p>
  
  <p>My model:</p>
  
  <pre><code>class Activity(models.Model):
      number = models.PositiveIntegerField(
          blank=True, null=True, help_text="Activity number. For record keeping only.")
      instructions = models.TextField()
      difficulty = models.ForeignKey(Difficulty)
      categories = models.ManyToManyField(Category)
      boosters = models.ManyToManyField(Booster)
  
      class Meta():
          verbose_name_plural = "Activities"
  </code></pre>
  
  <p>My serializer:</p>
  
  <pre><code>class ActivitySerializer(serializers.ModelSerializer):
  
      class Meta:
          model = Activity
  </code></pre>
  
  <p>And my view:   </p>
  
  <pre><code>class ActivityDetail(generics.RetrieveUpdateDestroyAPIView):
  
      queryset = Activity.objects.all()
      serializer_class = ActivityDetailSerializer
  </code></pre>
  
  <p>How can I get the model returned, with an additional field <code>user</code> such that my response looks like this:</p>
  
  <pre><code>{
      "id": 1, 
      "difficulty": 1, 
      "categories": [
          1
      ], 
      "boosters": [
          1
      ],
      "current_user": 1 //Current authenticated user here
  }
  </code></pre>