fix: apply column_tags when updating a view via ALTER

A V2 view with view_update_via_alter: true errored with "main is not
being called during running model" on a run whose only change was its
column tags: databricks__alter_view had no column_tags branch, so the
alter macro became a no-op that never called statement('main'). Add the
branch so the changed column tags are applied on the alter path, matching
the view create path and the table/incremental paths.

Closes #1525

Author: sd-db

CHANGELOG.md

@@ -5,6 +5,7 @@
 - Add catalogs.yml v2 support (requires `use_catalogs_v2: true` in dbt-core) ([1440](https://github.com/databricks/dbt-databricks/pull/1440))
 
 ### Fixes
+- Apply column-level `databricks_tags` when a Unity Catalog view is updated via `ALTER` (`view_update_via_alter: true`). A run whose only change was its column tags previously errored with `main is not being called during running model` because the `alter_view` path had no `column_tags` branch; it now applies the changed tags there, matching the view create path. ([#XXXX](https://github.com/databricks/dbt-databricks/pull/XXXX) closes [#1525](https://github.com/databricks/dbt-databricks/issues/1525))
 - Raise a `DbtRuntimeError` when a Python model job run terminates with a non-success `result_state` (e.g. `FAILED`/`TIMEDOUT`) instead of returning silently ([#1477](https://github.com/databricks/dbt-databricks/pull/1477))
 
 ### Under the Hood

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

@@ -8,6 +8,7 @@
   {% set tblproperties = changes.get("tblproperties") %}
   {% set query = changes.get("query") %}
   {% set column_comments = changes.get("column_comments") %}
+  {% set column_tags = changes.get("column_tags") %}
   {% if tags %}
     {{ apply_tags(target_relation, tags.set_tags) }}
   {% endif %}
@@ -20,4 +21,7 @@
   {% if column_comments %}
     {{ alter_column_comments(target_relation, column_comments.comments) }}
   {% endif %}
+  {% if column_tags %}
+    {{ apply_column_tags(target_relation, column_tags) }}
+  {% endif %}
 {% endmacro %}

tests/functional/adapter/column_tags/test_column_tags.py

@@ -116,6 +116,21 @@ class TestColumnTagsView(ColumnTagsMixin):
     relation_type = "view"
 
 
+@pytest.mark.skip_profile("databricks_cluster")
+class TestColumnTagsViewUpdateViaAlter(ColumnTagsMixin):
+    relation_type = "view"
+
+    @pytest.fixture(scope="class")
+    def project_config_update(self):
+        # With view_update_via_alter, a subsequent run whose only change is its column
+        # tags takes the alter_view path instead of a full replace; the changed tags
+        # must be applied there too.
+        return {
+            "flags": {"use_materialization_v2": True},
+            "models": {"+view_update_via_alter": True},
+        }
+
+
 @pytest.mark.skip_profile("databricks_cluster")
 class TestColumnTagsTableV1(ColumnTagsMixin):
     relation_type = "table"

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["apply_column_tags"] = Mock()
 
     def render_alter_view(self, template_bundle, changes):
         return self.run_macro(
@@ -84,3 +85,10 @@ def test_macros__alter_view_with_query(self, context, template_bundle):
         context["apply_tags"].assert_not_called()
         context["apply_tblproperties"].assert_not_called()
         context["alter_query"].assert_called_once()
+
+    def test_macros__alter_view_with_column_tags(self, context, template_bundle):
+        self.render_alter_view(template_bundle, {"column_tags": Mock()})
+        context["apply_tags"].assert_not_called()
+        context["apply_tblproperties"].assert_not_called()
+        context["alter_query"].assert_not_called()
+        context["apply_column_tags"].assert_called_once()