How to update audit column on an insert or update

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1

The problem I’m having

I am trying to implment a model that merges data into a target table. The target contains an audit field that is not in the source data: last_updated. I would like this field to be set to the current time whenever a row is inserted into or updated in the target.

The context of why I’m trying to do this

I want to reflect the last time a row was updated (or inserted).

What I’ve already tried

{{ 
    config(
        alias='dbt_scd_1_target',
        materialized='incremental',
        unique_key='my_bk',
        incremental_strategy='merge',
        merge_exclude_columns = ['last_updated'],
    ) 
}}

select 
    s.*,
    systimestamp() last_updated
from 
    dbt_scd_1_source s

This works for rows that are inserted, but not for those that are updated.