private void makeAttributeCombinerTransformIfAppropriate(SummaryField summaryField, Schema schema) {
        if (summaryField.getTransform() == SummaryTransform.NONE) {
            String sourceFieldName = summaryField.getSingleSource();
            ImmutableSDField source = schema.getField(sourceFieldName);
            if (source != null && isComplexFieldWithOnlyStructFieldAttributes(source)) {
                summaryField.setTransform(SummaryTransform.ATTRIBUTECOMBINER);
            }
        }
    }
If the source is a complex field with only struct field attributes then make this use the attribute combiner transform
config-model/src/main/java/com/yahoo/schema/processing/AdjustSummaryTransforms.java