title: context in nested serializers django rest framework
body: <p>If i have a nested serializer:</p>
  
  <pre><code>class ChildSerializer(ModelSerializer):
      class Meta:
          fields = ('c_name', )
          model = Child
  
  
  class ParentSerializer(ModelSerializer):
  
      child = ChildSerializer(many=True, read_only=True)
  
      class Meta:
          model = Parent
          fields = ('p_name', 'child')
  </code></pre>
  
  <p>And i want to access the context in the nested serializer, how can i do that? As far as i can tell, context isn't passed to the Child. </p>
  
  <p>I want to be able to implement a permission model per user on fields, for that i overridden the get_fields() method of the ModelSerializer:</p>
  
  <pre><code>def get_fields(self):
      fields = super().get_fields()
      ....
      for f in fields:
          if has_rights(self.context['request'].user, f, "read"):
              ret_val[f] = fields[f]
      ....
      return ret_val
  </code></pre>
  
  <p>Which works for regular serializers, but the context, and thus the request and user are not available when the nested child is passed to get_fields(). How do i access the context when the serializer is nested?</p>