[Backport 1.5.latest] Nicer error message for contracted model missing 'columns'

.changes/unreleased/Fixes-20230704-114752.yaml

@@ -0,0 +1,6 @@
+kind: Fixes
+body: Nicer error message if model with enforced contract is missing 'columns' specification
+time: 2023-07-04T11:47:52.976527+02:00
+custom:
+  Author: jtcohen6
+  Issue: "7943"

core/dbt/exceptions.py

@@ -2250,6 +2250,11 @@ def get_mismatches(self) -> agate.Table:
         return table_from_data_flat(mismatches_sorted, column_names)
 
     def get_message(self) -> str:
+        if not self.yaml_columns:
+            return (
+                "This model has an enforced contract, and its 'columns' specification is missing"
+            )
+
         table: agate.Table = self.get_mismatches()
         # Hack to get Agate table output as string
         output = io.StringIO()

core/dbt/include/global_project/macros/materializations/models/table/columns_spec_ddl.sql

@@ -33,10 +33,17 @@
   If any differences in name, data_type or number of columns exist between the two schemas, raises a compiler error
 #}
 {% macro assert_columns_equivalent(sql) %}
+
+  {#-- First ensure the user has defined 'columns' in yaml specification --#}
+  {%- set user_defined_columns = model['columns'] -%}
+  {%- if not user_defined_columns -%}
+      {{ exceptions.raise_contract_error([], []) }}
+  {%- endif -%}
+
   {#-- Obtain the column schema provided by sql file. #}
   {%- set sql_file_provided_columns = get_column_schema_from_query(sql, config.get('sql_header', none)) -%}
   {#--Obtain the column schema provided by the schema file by generating an 'empty schema' query from the model's columns. #}
-  {%- set schema_file_provided_columns = get_column_schema_from_query(get_empty_schema_sql(model['columns'])) -%}
+  {%- set schema_file_provided_columns = get_column_schema_from_query(get_empty_schema_sql(user_defined_columns)) -%}
 
   {#-- create dictionaries with name and formatted data type and strings for exception #}
   {%- set sql_columns = format_columns(sql_file_provided_columns) -%}

tests/functional/configs/test_contract_configs.py

@@ -471,3 +471,18 @@ def test__python_errors(self, project):
         exc_str = " ".join(str(err_info.value).split())
         expected_python_error = "Language Error: Expected 'sql' but found 'python'"
         assert expected_python_error in exc_str
+
+
+class TestModelContractMissingYAMLColumns:
+    @pytest.fixture(scope="class")
+    def models(self):
+        return {
+            "my_model.sql": my_model_contract_sql,
+        }
+
+    def test__missing_column_contract_error(self, project):
+        results = run_dbt(["run"], expect_pass=False)
+        expected_error = (
+            "This model has an enforced contract, and its 'columns' specification is missing"
+        )
+        assert expected_error in results[0].message