@Override
    protected void doXContent(XContentBuilder builder) throws IOException {
        builder.startObject("exists");
        builder.field("field", fieldName);
        builder.endObject();
    }
@see io.apiman.manager.api.es.util.AbstractQueryBuilder#doXContent(io.apiman.manager.api.es.util.XContentBuilder)
manager/api/es/src/main/java/io/apiman/manager/api/es/util/ExistsFilterBuilder.java