private void addDocumentFrequencies(List<Item> candidates, Ranking ranking) {
        for (Item  candidate : candidates) {
            TaggableItem t = (TaggableItem) candidate;
            var documentFrequency = t.getDocumentFrequency();
            if (documentFrequency.isEmpty()) continue;
            String name = "vespa.term." + t.getUniqueID() + ".docfreq";
            ranking.getProperties().put(name, String.valueOf(documentFrequency.get().frequency()));
            ranking.getProperties().put(name, String.valueOf(documentFrequency.get().count()));
        }
    }
Prepares this for binary serialization. For internal use.
container-search/src/main/java/com/yahoo/search/query/Model.java