Add test for 'a has fewer rows than b'

README.md

@@ -132,6 +132,19 @@ models:
 
 ```
 
+#### fewer_rows_than ([source](macros/schema_tests/fewer_rows_than.sql))
+This schema test asserts that this model has fewer rows than the referenced model.
+
+Usage:
+```yaml
+version: 2
+
+models:
+  - name: model_name
+    tests:
+      - dbt_utils.fewer_rows_than:
+          compare_model: ref('other_table_name')
+
 #### equality ([source](macros/schema_tests/equality.sql))
 This schema test asserts the equality of two relations. Optionally specify a subset of columns to compare.
 

integration_tests/data/schema_tests/data_test_fewer_rows_than_table_1.csv

@@ -0,0 +1,4 @@
+field
+1
+2
+3

integration_tests/data/schema_tests/data_test_fewer_rows_than_table_2.csv

@@ -0,0 +1,5 @@
+field
+1
+2
+3
+4

integration_tests/models/schema_tests/test_fewer_rows_than.sql

@@ -0,0 +1,9 @@
+with data as (
+
+    select * from {{ ref('data_test_fewer_rows_than_table_1') }}
+
+)
+
+select
+    field
+from data

macros/schema_tests/fewer_rows_than.sql

@@ -0,0 +1,39 @@
+{% macro test_fewer_rows_than(model) %}
+
+{% set compare_model = kwargs.get('compare_model', kwargs.get('arg')) %}
+
+with a as (
+
+    select count(*) as count_ourmodel from {{ model }}
+
+),
+b as (
+
+    select count(*) as count_comparisonmodel from {{ compare_model }}
+
+),
+counts as (
+
+    select
+        (select count_ourmodel from a) as count_model_with_fewer_rows,
+        (select count_comparisonmodel from b) as count_model_with_more_rows
+
+),
+final as (
+
+    select 
+        case
+            -- fail the test if we have more rows than the reference model and return the row count delta
+            when count_model_with_fewer_rows > count_model_with_more_rows then (count_model_with_fewer_rows - count_model_with_more_rows)
+            -- fail the test if they are the same number
+            when count_model = count_comparison then 1
+            -- pass the test if the delta is positive (i.e. return the number 0)
+            else 0 
+    end as row_count_delta
+    from counts
+
+)
+
+select row_count_delta from final
+
+{% endmacro %}