Extend 'expression_is_true' test to support subset asertions

[dcereijodo]

expression_is_true test is really useful, but sometimes I am missing the option of using this test to assert on a subset of the rows of the model. For example, I would like to be able to assert that price is higher than 5K if product_type is a car. For these reason I ended up creating a custom test for this like the one below, thought it feels like this kind of functionality could be in the expression_is_true test already.

{#
  Checks that an expression is true on those rows that meet a previous condition
  This is different from the 'expression_is_true' test from dbt-utils in that can
  limit the input dataset to be tested by using a 'condition'

  Arguments:
    - model: the name of the model to be tested
    - expression: the expression to be asserted in the data
    - condition: filters to a subset of the model rows
#}
{% macro test_true_on_condition(model, expression, condition) %}

with meet_condition as (
  select * from {{ model }} where {{ condition }}
)
select count(*)
from meet_condition
where not({{ expression }})

{% endmacro %}

Proposal: extend the expression_is_true test with an optional parameter condition that takes a none default value. When nothing is passed in the condition argument the expression is tested against the whole model. When a condition is passed the the model is subset by the condition prior to asserting the expression.

I'd be happy to send a PR this fits.

[drewbanin]

Hey @dcereijodo - cool idea! One way to hack this is with a statement like:

case when product_type = 'car' then price > 5000 else true end

I like that this macro is flexible, but i also think that using an expression like this is less semantically meaningful (and harder to write!) than the addition you're describing.

So, sure, if you're interested in augmenting this test then feel free to send along a PR :)