// serialize key
String serializedKey = keyAdapter.serialize(e.getKey(), config, null);
// if new node
if (prev.get(e.getKey()) == null) {
ConfigWriter elementWriter = collectionWriter.getCollectionElementWriter(fieldAnno.mapKey(), serializedKey, field);
// serialize
Object serialized = valueAdapter.serialize(e.getValue(), config, field);
valueAdapter.write(serialized, config, elementWriter, field);
elementWriter.flushWriter();
}