Fix: Handle view alterations and existing table replacements

This commit addresses the following:

- Modifies the `create view` macro to use `alter view` when the view already exists, preventing errors during re-materialization.
- Updates the view materialization logic to correctly handle scenarios where a table with the same name exists. It now renames the existing table to a backup before creating the view.
- Adds a test case to verify that materializing an existing view updates its definition correctly, including column changes.

Author: axellpadilla

dbt/include/sqlserver/macros/materializations/models/view/view.sql

@@ -36,22 +36,23 @@
   -- `BEGIN` happens here:
   {{ run_hooks(pre_hooks, inside_transaction=True) }}
 
-  -- build model
-  {% call statement('main') -%}
-    {{ get_create_view_as_sql(intermediate_relation, sql) }}
-  {%- endcall %}
-
-  -- cleanup
-  -- move the existing view out of the way
-  {% if existing_relation is not none %}
+  -- move the existing table if not view out of the way
+  {% if existing_relation is not none and existing_relation.type != 'view' %}
      /* Do the equivalent of rename_if_exists. 'existing_relation' could have been dropped
         since the variable was first set. */
     {% set existing_relation = load_cached_relation(existing_relation) %}
     {% if existing_relation is not none %}
         {{ adapter.rename_relation(existing_relation, backup_relation) }}
     {% endif %}
   {% endif %}
-  {{ adapter.rename_relation(intermediate_relation, target_relation) }}
+
+  -- build model
+  {% call statement('main') -%}
+    {# The underlying macro sqlserver__create_view_as handles CREATE vs ALTER logic #}
+    {{ get_create_view_as_sql(target_relation, sql) }}
+  {%- endcall %}
+
+  -- cleanup
 
   {% set should_revoke = should_revoke(existing_relation, full_refresh_mode=True) %}
   {% do apply_grants(target_relation, grant_config, should_revoke=should_revoke) %}

dbt/include/sqlserver/macros/relations/views/create.sql

@@ -6,8 +6,14 @@
         {{ get_assert_columns_equivalent(sql) }}
     {%- endif %}
 
+    {% set existing_relation = load_cached_relation(relation) %}
+
     {% set query %}
-        create view {{ relation.include(database=False) }} as {{ sql }};
+        {% if existing_relation is not none %}
+            alter view {{ relation.include(database=False) }} as {{ sql }};
+        {% else %}
+            create view {{ relation.include(database=False) }} as {{ sql }};
+        {% endif %}
     {% endset %}
 
     {% set tst %}

tests/functional/adapter/dbt/test_basic.py

@@ -1,3 +1,5 @@
+import os
+
 import pytest
 from dbt.tests.adapter.basic.test_adapter_methods import BaseAdapterMethod
 from dbt.tests.adapter.basic.test_base import BaseSimpleMaterializations
@@ -9,10 +11,64 @@
 from dbt.tests.adapter.basic.test_singular_tests_ephemeral import BaseSingularTestsEphemeral
 from dbt.tests.adapter.basic.test_snapshot_check_cols import BaseSnapshotCheckCols
 from dbt.tests.adapter.basic.test_snapshot_timestamp import BaseSnapshotTimestamp
+from dbt.tests.util import run_dbt
 
 
 class TestSimpleMaterializations(BaseSimpleMaterializations):
-    pass
+
+    def test_existing_view_materialization(self, project, models):
+        """Test that materializing an existing view works correctly."""
+        # Create a temporary model file directly in the project
+        model_path = os.path.join(project.project_root, "models", "view_model_exists.sql")
+
+        # Write the initial model without the value column
+        with open(model_path, "w") as f:
+            f.write(
+                """
+                {{ config(materialized='view') }}
+                select
+                1 as id
+                {% if var('include_value_column', false) %}
+                , 2 as value
+                {% endif %}
+            """
+            )
+
+        # First run to create the view without the extra column
+        results = run_dbt(["run", "-m", "view_model_exists"])
+        assert len(results) == 1
+
+        # Generate catalog to get column information
+        catalog = run_dbt(["docs", "generate"])
+
+        # Check columns in the catalog
+        node_id = "model.base.view_model_exists"
+        assert node_id in catalog.nodes
+
+        # Get columns from the catalog
+        columns = catalog.nodes[node_id].columns
+        column_names = [name.lower() for name in columns.keys()]
+
+        # Verify only the id column exists
+        assert "id" in column_names
+        assert "value" not in column_names
+
+        # Second run with a variable to include the extra column
+        results = run_dbt(
+            ["run", "-m", "view_model_exists", "--vars", '{"include_value_column": true}']
+        )
+        assert len(results) == 1
+
+        # Generate catalog again to get updated column information
+        catalog = run_dbt(["docs", "generate"])
+
+        # Get updated columns from the catalog
+        columns = catalog.nodes[node_id].columns
+        column_names = [name.lower() for name in columns.keys()]
+
+        # Verify both columns exist now
+        assert "id" in column_names
+        assert "value" in column_names
 
 
 class TestSingularTests(BaseSingularTests):