code: public M keep(String attr) {
  		if (attrs.containsKey(attr)) {	// prevent put null value to the newColumns
  			Object keepIt = attrs.get(attr);
  			boolean keepFlag = _getModifyFlag().contains(attr);
  			attrs.clear();
  			_getModifyFlag().clear();
  			attrs.put(attr, keepIt);
  			if (keepFlag)
  				_getModifyFlag().add(attr);
  		}
  		else {
  			attrs.clear();
  			_getModifyFlag().clear();
  		}
  		return (M)this;
  	}
docs: Keep attribute of this model and remove other attributes.
  @param attr the attribute name of the model
  @return this model