code: public Record keep(String... columns) {
  		if (columns != null && columns.length > 0) {
  			Map<String, Object> newColumns = new HashMap<String, Object>(columns.length);	// getConfig().containerFactory.getColumnsMap();
  			for (String c : columns)
  				if (this.getColumns().containsKey(c))	// prevent put null value to the newColumns
  					newColumns.put(c, this.getColumns().get(c));
  			
  			this.getColumns().clear();
  			this.getColumns().putAll(newColumns);
  		}
  		else
  			this.getColumns().clear();
  		return this;
  	}
docs: Keep columns of this record and remove other columns.
  @param columns the column names of the record