code: public RowId getRowId(String field) {
  		Object obj = this.get(field);
  		if (null == obj) {
  			return null;
  		}
  		if (obj instanceof RowId) {
  			return (RowId) obj;
  		}
  		throw new DbRuntimeException("Value of field [{}] is not a rowid!", field);
  	}
docs: 获得rowid
  
  @param field rowid属性名
  @return RowId