'Indicates whether or not this value has changed.
:rtype: boolean'

@property
def changed(self):

    if self.explicit:
        return (self.value != self.previous_value)
    if isinstance(self.column, BaseContainerColumn):
        default_value = self.column.get_default()
        if self.column._val_is_null(default_value):
            return ((not self.column._val_is_null(self.value)) and (self.value != self.previous_value))
        elif (self.previous_value is None):
            return (self.value != default_value)
        return (self.value != self.previous_value)
    return False
