fix: missing column comments after ALTER VIEW AS

CHANGELOG.md

@@ -16,6 +16,7 @@
 - Only emit `INSERT ... BY NAME` in the `replace_where`/`microbatch` strategies on DBR 18.0+ (and SQL warehouses), since older clusters reject the `BY NAME ... REPLACE WHERE` combination with a parse error ([1539](https://github.com/databricks/dbt-databricks/pull/1539) closes [#1532](https://github.com/databricks/dbt-databricks/issues/1532))
 - Fix materialized views always rebuilding because Databricks-internal `tblproperties` were read as configuration drift; the diff now compares only the configured properties ([#1350](https://github.com/databricks/dbt-databricks/pull/1350) closes [#1314](https://github.com/databricks/dbt-databricks/issues/1314)).
 - Stop metric views with `view_update_via_alter` from re-issuing a redundant `ALTER VIEW ... AS` on every run ([#1546](https://github.com/databricks/dbt-databricks/pull/1546))
+- Fix column comments being permanently dropped from views when `view_update_via_alter` issues `ALTER VIEW AS`; reapply persisted column comments after the query update ([#1357](https://github.com/databricks/dbt-databricks/issues/1357))
 
 ### Under the Hood
 

dbt/include/databricks/macros/relations/view/alter.sql

@@ -17,6 +17,12 @@
   {% endif %}
   {% if query %}
     {{ alter_query(target_relation, query.query) }}
+    {% if config.persist_column_docs() and model.columns %}
+      {#-- ALTER VIEW AS <query> wipes all column comments, so reapply them here. --#}
+      {%- set existing_columns = adapter.get_columns_in_relation(target_relation) -%}
+      {%- set columns_to_persist = adapter.get_persist_doc_columns(existing_columns, model.columns) -%}
+      {{ alter_column_comment(target_relation, columns_to_persist) }}
+    {% endif %}
   {% endif %}
   {% if column_comments %}
     {{ alter_column_comments(target_relation, column_comments.comments) }}

tests/functional/adapter/views/test_views.py

@@ -90,6 +90,21 @@ def test_view_update_with_column_comments(self, project):
         assert results[0][2] == "This is an id column"
 
 
+class BaseUpdateQueryPreservesColumnComments(BaseUpdateView):
+    """Regression for #1357: ALTER VIEW AS wipes column comments; they must be reapplied."""
+
+    def test_view_update_query_preserves_column_comments(self, project):
+        util.run_dbt(["build"])
+        util.write_file(fixtures.altered_view_sql, "models", "initial_view.sql")
+        util.run_dbt(["run"])
+
+        results = project.run_sql(
+            "describe extended {database}.{schema}.initial_view",
+            fetch="all",
+        )
+        assert results[0][2] == "This is the id column"
+
+
 class BaseRemoveTags(BaseUpdateView):
     def test_view_update_remove_tags(self, project):
         util.run_dbt(["build"])
@@ -213,6 +228,22 @@ def project_config_update(self):
         }
 
 
+@pytest.mark.skip_profile("databricks_cluster")
+class TestUpdateViewViaAlterQueryPreservesColumnComments(BaseUpdateQueryPreservesColumnComments):
+    @pytest.fixture(scope="class")
+    def project_config_update(self):
+        return {
+            "flags": {"use_materialization_v2": True},
+            "models": {
+                "+view_update_via_alter": True,
+                "+persist_docs": {
+                    "relation": True,
+                    "columns": True,
+                },
+            },
+        }
+
+
 @pytest.mark.skip_profile("databricks_cluster")
 class TestUpdateViewViaAlterRemoveTags(BaseRemoveTags):
     @pytest.fixture(scope="class")

tests/unit/macros/relations/test_view_macros.py

@@ -52,6 +52,7 @@ def mocks(self, context):
         context["apply_tags"] = Mock()
         context["apply_tblproperties"] = Mock()
         context["alter_query"] = Mock()
+        context["alter_column_comment"] = Mock()
         context["apply_column_tags"] = Mock()
 
     def render_alter_view(self, template_bundle, changes):
@@ -86,6 +87,15 @@ def test_macros__alter_view_with_query(self, context, template_bundle):
         context["apply_tblproperties"].assert_not_called()
         context["alter_query"].assert_called_once()
 
+    def test_macros__alter_view_with_query_reapplies_column_comments(
+        self, context, template_bundle
+    ):
+        context["config"].persist_column_docs = Mock(return_value=True)
+        context["model"].columns = {"id": Mock()}
+        self.render_alter_view(template_bundle, {"query": Mock()})
+        context["alter_query"].assert_called_once()
+        context["alter_column_comment"].assert_called_once()
+
     def test_macros__alter_view_with_column_tags(self, context, template_bundle):
         column_tags = Mock()
         self.render_alter_view(template_bundle, {"column_tags": column_tags})