Check for subset of columns (Close dbt-labs#785)

integration_tests/models/generic_tests/equality_less_columns.sql

@@ -0,0 +1,9 @@
+with data as (
+
+    select * from {{ ref('data_test_equality_b') }}
+
+)
+
+select
+    col_a, col_b
+from data

integration_tests/models/generic_tests/schema.yml

@@ -250,3 +250,10 @@ models:
       - dbt_utils.fewer_rows_than:
           compare_model: ref('data_test_fewer_rows_than_table_2')
           group_by_columns: ['col_a']
+
+  - name: equality_less_columns
+    tests:
+      - dbt_utils.equality:
+          compare_model: ref('data_test_equality_a')
+          error_if: "<1" #sneaky way to ensure that the test is returning failing rows
+          warn_if: "<0"

integration_tests/models/sql/test_union.sql

@@ -2,7 +2,7 @@
 select
     id,
     name,
-    favorite_color
+    favorite_color,
+    favorite_number
 
 from {{ ref('test_union_base') }}
-

macros/generic_tests/equality.sql

@@ -23,9 +23,22 @@
     {{ return('') }}
 {% endif %}
 
+
+
 -- setup
 {%- do dbt_utils._is_relation(model, 'test_equality') -%}
 
+{# Ensure there are no extra columns in the compare_model vs model #}
+{%- if not compare_columns and not ignore_columns -%}
+    {%- do dbt_utils._is_ephemeral(model, 'test_equality') -%}
+    {%- set compare_columns_set = set(adapter.get_columns_in_relation(model) | map(attribute='quoted'))  -%}
+    {%- do dbt_utils._is_ephemeral(model, 'test_equality') -%}
+    {%- set compare_model_columns_set = set(adapter.get_columns_in_relation(compare_model) | map(attribute='quoted')) -%}
+    {% if compare_columns_set != compare_model_columns_set %}
+        {{ return("select 1, 'b_minus_a' as which_diff from " ~ compare_model) }}
+    {% endif %}
+{% endif %}
+
 {%- if not precision -%}
     {#-
         If the compare_cols arg is provided, we can run this test without querying the