code: public DBObject getLinkedObject(String owningObjID,
                                      String linkFieldName,
                                      String linkedObjID) {
          Map<String, Map<String, DBObject>> objMap = m_linkedObjectMap.get(owningObjID);
          if (objMap != null) {
              Map<String, DBObject> linkMap = objMap.get(linkFieldName);
              if (linkMap != null) {
                  return linkMap.get(linkedObjID);
              }
          }
          return null;
      }
docs: Get the {@link DBObject} that is linked to the given "owner" object ID via the given
  link name with the given "linked" object ID. If the requested object was returned by
  the query, the DBObject minimally contains an object ID but only the additional
  scalar and/or link fields requested by the query. The same object may be linked to
  multiple owning objects with different fields, as directed by the query.
  
  @param owningObjID   Object ID of the object that owns the given link field.
  @param linkFieldName Name of link field.
  @param linkedObjID   Object ID of the linked object to fetch.
  @return              The {@link DBObject} of the linked object if it was returned by
  query, otherwise null.