code: public UNode toDoc() {
          UNode resultNode = UNode.createMapNode("doc");
          for (String fieldName : getUpdatedFieldNames()) {
              leafFieldtoDoc(resultNode, fieldName);
          }
          return resultNode;
      }
docs: Serialize this DBObject into a {@link UNode} tree, returning the root node. The
  tree returned can then be turned into JSON or XML by calling {@link UNode#toJSON()}
  or {@link UNode#toXML()}. This method ignores the structure of group fields: all
  fields are returned in a "flat" structure. For example, a UNode tree converted to
  JSON might look like:
  <pre>
  {"doc": {
  "_ID": "28cn2812ur",
  "_table": "Message",
  "Subject": "Concrete slabs",
  "Tags": {
  "add": ["Confidential", "Priority"]   // MV values are sorted
  },
  "Sender": ["1dj2r4j1l"]
  }}
  </pre>
  
  @return The root node of a UNode tree.
  @see    #toGroupedDoc(TableDefinition)