code: public String getIdProperty(TableRef tableRef) {
          TableMapping tableMapping = mappedClasses.get(tableRef);
          if (tableMapping == null) {
              return null;
          }
          ColumnMetaData identifierColumn = tableMapping.getIdentifierColumn();
          return tableMapping.getColumnMapping(identifierColumn).getPropertyName();
      }
docs: Returns the name of the Identifier property
  
  @param tableRef the <code>TableRef</code> for the table
  @return the name of the identifier property, or null if the table specified by the <code>tableRef</code>
  parameter is not mapped
  @throws IllegalStateException if the map() method has not been invoked first.