docs: def add_data(self, request, pk=None):
          """Add data to collection."""
          collection = self.get_object()
  
          if 'ids' not in request.data:
              return Response({"error": "`ids`parameter is required"}, status=status.HTTP_400_BAD_REQUEST)
  
          missing = []
          for data_id in request.data['ids']:
              if not Data.objects.filter(pk=data_id).exists():
                  missing.append(data_id)
  
          if missing:
              return Response(
                  {"error": "Data objects with following ids are missing: {}".format(', '.join(missing))},
                  status=status.HTTP_400_BAD_REQUEST)
  
          for data_id in request.data['ids']:
              collection.data.add(data_id)
  
          return Response()