title: customize Django Rest Framework serializers output?
body: <p>I have a Django model like this:</p>
  
  <pre><code>class Sections(models.Model):
      section_id = models.CharField(max_length=127, null=True, blank=True)
      title = models.CharField(max_length=255)
      description = models.TextField(null=True, blank=True)
  
  class Risk(models.Model):
      title = models.CharField(max_length=256, null=False, blank=False)
      section = models.ForeignKey(Sections, related_name='risks')
  
  class Actions(models.Model):
      title = models.CharField(max_length=256, null=False, blank=False)
      section = models.ForeignKey(Sections, related_name='actions')
  </code></pre>
  
  <p>And serializers like that :</p>
  
  <pre><code>class RiskSerializer(serializers.ModelSerializer):
      class Meta:
          model = Risk
          fields = ('id', 'title',)
  
  class ActionsSerializer(serializers.ModelSerializer):
      class Meta:
          model = Actions
          fields = ('id', 'title',)
  
  class RiskActionPerSectionsSerializer(serializers.ModelSerializer):
     risks = RiskSerializer(many=True, read_only=True)
     actions = ActionsSerializer(many=True, read_only=True)
  
     class Meta:
          model = Sections
          fields = ('section_id', 'risks', 'actions')
          depth = 1
  </code></pre>
  
  <p>When accessing the RiskActionPerSectionSerializer over a view, I get the following output:</p>
  
  <pre><code>[
  {
      "actions": [
          {
              "id": 2, 
              "title": "Actions 2"
          }, 
          {
              "id": 1, 
              "title": "Actions 1"
          }
      ], 
      "risks": [
          {
              "id": 2, 
              "title": "Risk 2"
          }, 
          {
              "id": 1, 
              "title": "Risk 1"
          }
      ], 
      "section_id": "section 1"
  }
  .....
  ]
  </code></pre>
  
  <p>It s fine but I would prefer to have that :</p>
  
  <pre><code>{
  "section 1": {
      "risks": [
          {
            "id": 2,
            "title": "Risk 2"
          }, 
          {
            "id": 1,
            "title": "Risk 1"
          }
      ],
      "actions": [
          {
            "id": 2,
            "title": "Actions 2"
          }, 
          {
            "id": 1,
            "title": "Actions 1"
          }
     ]
  }
  }
  </code></pre>
  
  <p>How can I do that with Django Rest Framework ?</p>