code: public function getOldAttribute($name)
      {
          return isset($this->_oldAttributes[$name]) ? $this->_oldAttributes[$name] : null;
      }
docs: Returns the old value of the named attribute.
  If this record is the result of a query and the attribute is not loaded,
  `null` will be returned.
  @param string $name the attribute name
  @return mixed the old attribute value. `null` if the attribute is not loaded before
  or does not exist.
  @see hasAttribute()