private void registerReferenceDocumentType() {
        DocumentType docTypeWithRef = new DocumentType("testrefs");
        ReferenceDataType type = ReferenceDataType.createWithInferredId(types.getDocumentType("smoke"));
        docTypeWithRef.addField(new Field("ref_field", type));
        types.registerDocumentType(docTypeWithRef);
    }
Functional tests for com.yahoo.document.json.JsonWriter.

@author Steinar Knutsen
document/src/test/java/com/yahoo/document/json/JsonWriterTestCase.java