fix(fabric): Alter table workaround

sqlmesh/core/engine_adapter/fabric.py

@@ -13,6 +13,8 @@
 )
 from sqlmesh.utils.errors import SQLMeshError
 from sqlmesh.utils.connection_pool import ConnectionPool
+from sqlmesh.core.schema_diff import TableAlterOperation
+from sqlmesh.utils import random_id
 
 
 logger = logging.getLogger(__name__)
@@ -153,6 +155,113 @@ def set_current_catalog(self, catalog_name: str) -> None:
                 f"Unable to switch catalog to {catalog_name}, catalog ended up as {catalog_after_switch}"
             )
 
+    def alter_table(
+        self, alter_expressions: t.Union[t.List[exp.Alter], t.List[TableAlterOperation]]
+    ) -> None:
+        """
+        Applies alter expressions to a table. Fabric has limited support for ALTER TABLE,
+        so this method implements a workaround for column type changes.
+        This method is self-contained and sets its own catalog context.
+        """
+        if not alter_expressions:
+            return
+
+        # Get the target table from the first expression to determine the correct catalog.
+        first_op = alter_expressions[0]
+        expression = first_op.expression if isinstance(first_op, TableAlterOperation) else first_op
+        if not isinstance(expression, exp.Alter) or not expression.this.catalog:
+            # Fallback for unexpected scenarios
+            logger.warning(
+                "Could not determine catalog from alter expression, executing with current context."
+            )
+            super().alter_table(alter_expressions)
+            return
+
+        target_catalog = expression.this.catalog
+        self.set_current_catalog(target_catalog)
+
+        with self.transaction():
+            for op in alter_expressions:
+                expression = op.expression if isinstance(op, TableAlterOperation) else op
+
+                if not isinstance(expression, exp.Alter):
+                    self.execute(expression)
+                    continue
+
+                for action in expression.actions:
+                    table_name = expression.this
+
+                    table_name_without_catalog = table_name.copy()
+                    table_name_without_catalog.set("catalog", None)
+
+                    is_type_change = isinstance(action, exp.AlterColumn) and action.args.get(
+                        "dtype"
+                    )
+
+                    if is_type_change:
+                        column_to_alter = action.this
+                        new_type = action.args["dtype"]
+                        temp_column_name_str = f"{column_to_alter.name}__{random_id(short=True)}"
+                        temp_column_name = exp.to_identifier(temp_column_name_str)
+
+                        logger.info(
+                            "Applying workaround for column '%s' on table '%s' to change type to '%s'.",
+                            column_to_alter.sql(),
+                            table_name.sql(),
+                            new_type.sql(),
+                        )
+
+                        # Step 1: Add a temporary column.
+                        add_column_expr = exp.Alter(
+                            this=table_name_without_catalog.copy(),
+                            kind="TABLE",
+                            actions=[
+                                exp.ColumnDef(this=temp_column_name.copy(), kind=new_type.copy())
+                            ],
+                        )
+                        add_sql = self._to_sql(add_column_expr)
+                        self.execute(add_sql)
+
+                        # Step 2: Copy and cast data.
+                        update_sql = self._to_sql(
+                            exp.Update(
+                                this=table_name_without_catalog.copy(),
+                                expressions=[
+                                    exp.EQ(
+                                        this=temp_column_name.copy(),
+                                        expression=exp.Cast(
+                                            this=column_to_alter.copy(), to=new_type.copy()
+                                        ),
+                                    )
+                                ],
+                            )
+                        )
+                        self.execute(update_sql)
+
+                        # Step 3: Drop the original column.
+                        drop_sql = self._to_sql(
+                            exp.Alter(
+                                this=table_name_without_catalog.copy(),
+                                kind="TABLE",
+                                actions=[exp.Drop(this=column_to_alter.copy(), kind="COLUMN")],
+                            )
+                        )
+                        self.execute(drop_sql)
+
+                        # Step 4: Rename the temporary column.
+                        old_name_qualified = f"{table_name_without_catalog.sql(dialect=self.dialect)}.{temp_column_name.sql(dialect=self.dialect)}"
+                        new_name_unquoted = column_to_alter.sql(
+                            dialect=self.dialect, identify=False
+                        )
+                        rename_sql = f"EXEC sp_rename '{old_name_qualified}', '{new_name_unquoted}', 'COLUMN'"
+                        self.execute(rename_sql)
+                    else:
+                        # For other alterations, execute directly.
+                        direct_alter_expr = exp.Alter(
+                            this=table_name_without_catalog.copy(), kind="TABLE", actions=[action]
+                        )
+                        self.execute(direct_alter_expr)
+
 
 class FabricHttpClient:
     def __init__(self, tenant_id: str, workspace_id: str, client_id: str, client_secret: str):

tests/core/engine_adapter/test_fabric.py

@@ -91,6 +91,61 @@ def test_replace_query(adapter: FabricEngineAdapter, mocker: MockerFixture):
     ]
 
 
+def test_alter_table_column_type_workaround(adapter: FabricEngineAdapter, mocker: MockerFixture):
+    """
+    Tests the alter_table method's workaround for changing a column's data type.
+    """
+    # Mock set_current_catalog to avoid connection pool side effects
+    set_catalog_mock = mocker.patch.object(adapter, "set_current_catalog")
+    # Mock random_id to have a predictable temporary column name
+    mocker.patch("sqlmesh.core.engine_adapter.fabric.random_id", return_value="abcdef")
+
+    alter_expression = exp.Alter(
+        this=exp.to_table("my_db.my_schema.my_table"),
+        actions=[
+            exp.AlterColumn(
+                this=exp.to_column("col_a"),
+                dtype=exp.DataType.build("BIGINT"),
+            )
+        ],
+    )
+
+    adapter.alter_table([alter_expression])
+
+    set_catalog_mock.assert_called_once_with("my_db")
+
+    expected_calls = [
+        "ALTER TABLE [my_schema].[my_table] ADD [col_a__abcdef] BIGINT;",
+        "UPDATE [my_schema].[my_table] SET [col_a__abcdef] = CAST([col_a] AS BIGINT);",
+        "ALTER TABLE [my_schema].[my_table] DROP COLUMN [col_a];",
+        "EXEC sp_rename 'my_schema.my_table.col_a__abcdef', 'col_a', 'COLUMN'",
+    ]
+
+    assert to_sql_calls(adapter) == expected_calls
+
+
+def test_alter_table_direct_alteration(adapter: FabricEngineAdapter, mocker: MockerFixture):
+    """
+    Tests the alter_table method for direct alterations like adding a column.
+    """
+    set_catalog_mock = mocker.patch.object(adapter, "set_current_catalog")
+
+    alter_expression = exp.Alter(
+        this=exp.to_table("my_db.my_schema.my_table"),
+        actions=[exp.ColumnDef(this=exp.to_column("new_col"), kind=exp.DataType.build("INT"))],
+    )
+
+    adapter.alter_table([alter_expression])
+
+    set_catalog_mock.assert_called_once_with("my_db")
+
+    expected_calls = [
+        "ALTER TABLE [my_schema].[my_table] ADD [new_col] INT;",
+    ]
+
+    assert to_sql_calls(adapter) == expected_calls
+
+
 def test_merge_pandas(
     make_mocked_engine_adapter: t.Callable, mocker: MockerFixture, make_temp_table_name: t.Callable
 ):