Auto update timestamp on update - python-3.x

Question

I have created a table where its following code is:
CREATE TABLE products (
id SERIAL,
store_id INTEGER NOT NULL,
url TEXT UNIQUE,
visible BOOLEAN,
added_date timestamp without time zone NOT NULL DEFAULT NOW(),
PRIMARY KEY(id, store_id)
);
and together with Peewee and I did figure out that on update, the timestamp doesn't automatically updates whenever I do an update on a row by doing e.g.
class Products(Model):
id = IntegerField(column_name='id')
store_id = TextField(column_name='store_id')
url = TextField(column_name='url')
visible = BooleanField(column_name='visible')
added_date = TimestampField(column_name='added_date')
class Meta:
database = postgres_pool
db_table = "products"
#classmethod
def on_exists(cls, pageData):
try:
if exists := cls.select().where((cls.store_id == Stores.store_id) & (cls.url == pageData.url)).exists():
# On update execution it should refresh the added_time to current time
cls.update(visible=False).where((cls.store_id == Stores.store_id) & (cls.url == pageData.url)).execute()
return exists
except Products.DoesNotExist:
return None
However my problem is that everytime I am using this command and successfully update. the added_time timestamp does not seem to update its timestamp. My question is, how can it automatically update its timestamp on update?

Answer 1

This is a job for a trigger:
CREATE FUNCTION upd_ts() RETURNS trigger
LANGUAGE plpgsql AS
$$BEGIN
NEW.added_date = current_timestamp;
RETURN NEW;
END;$$;
CREATE TRIGGER upd_ts BEFORE UPDATE ON products
FOR EACH ROW EXECUTE PROCEDURE upd_ts();
The trigger will fire before the rows are modified and change the row about to be inserted.
See the documentation for all the details.