title: How to pass flag to child serializer in DRF
body: <p>I have a nested serializer and I want to pass Parent serializer data to the child. But I don't understand how can I do this. I want to do something like this:</p>
  
  <pre><code>class BookingSerializer(serializers.ModelSerializer):
      use_additional_fields = serializers.BooleanField()
      persons = PersonSerializer(many=True)
  
  
  
  class PersonSerializer(serializers.ModelSerializer):
  
      def validate_date_of_birth(self, value):
         if parent.use_additional_fields and not value:
            raise serializers.ValidationError(_('Date of birth is required'))
         return value
  
      class Meta:
          model = Person
          exclude = ('phone', 'date_of_birth')
  </code></pre>
  
  <p>So if user select <code>use_additional_fields</code> in parent serializer, then some of my fields in child serializers should be required</p>