Oh this is really cool! As I just mentioned on your other PR, I'm going to do a big review on Friday!

I have a related thought bouncing around in my head:

What if we renamed the expression_is_true test to assert_where_clause (we have a pattern for deprecations) and created a new test assert_having_clause, something like this:

{% macro test_assert_having_clause(model, condition='true') %}

{% set expression = kwargs.get('expression', kwargs.get('arg')) %}

with

validation_errors as (

    select
        {{ column_name }}
    from meet_condition
    group by 1
    having not({{ expression }})

)

select count(*)
from validation_errors

{% endmacro %}

Then you could call it like:

version: 2

models:
  - name: orders_snapshot
    columns:
      - name: 
        tests:
          - assert_having_clause: "sum(case when is_current_version then 1 end) = 1"

I could imagine it being useful in other spots, e.g. does a ledger of transactions result in a positive entry:

version: 2

models:
  - name: transactions
    columns:
      - name: order_id
        tests:
          - assert_having_clause: "sum(amount) > 0"

Anyway, this is just a side-thought, and definitely not required for this PR at all, but wanted to chat through the thought with someone who is also thinking about this!

LOVE the assert_having_clause idea! I can think of a few places we'd use it.

I was just looking for something like this today. What do we need to do to get this one over the finish line?

Reopened as #313 so I could make changes