How to have a field be auto set every time the model is modified?

[Thermatix]

I can have a field for setting the creation date:

#[ormlite(default_value = "chrono::Utc::now()")]
pub updated_at: Option<DateTime<Utc>>,

but how can I have a field that is auto set every time the model is modified, I.e. an updated_at field?