private static boolean isAttribute(Field field) {
        SDField sdField = (SDField) field; // Ugly, but SDDocumentType only expose the fields as the super class Field
        return sdField.doesAttributing();
    }
Resolves all document references in the search definitions

Iterates through all document fields having a {@link NewDocumentReferenceDataType} and uses {@link NewDocumentReferenceDataType#getTargetTypeName()}
to determine the referenced document. This information is aggregated into a {@link DocumentReferences} object.

@author bjorncs
config-model/src/main/java/com/yahoo/schema/DocumentReferenceResolver.java