`'tmp_relation' is undefined` when altering a column's datatype on an incremental model

[baldwicc]

We're seeing error messages in v1.7.3 when the datatype of a column is altered (but the column name remains the same).

04:07:16  Running with dbt=1.7.15
04:07:16  Registered adapter: synapse=1.7.3

12:34:56    Compilation Error in model example_model_name (models/xxx/example_model_name.sql)
  'tmp_relation' is undefined
  
  > in macro alter_column_type (macros/adapters/columns.sql)
  > called by macro materialization_incremental_default (macros/materializations/models/incremental/incremental.sql)
  > called by model example_model_name (models/xxx/example_model_name.sql)

Root cause appears to be the upstream fabric adaptor not supporting ALTER TABLE, but also having it's own bug in this macro where tmp_relation is indeed never defined:

dbt-msft/dbt-sqlserver:
https://github.com/dbt-msft/dbt-sqlserver/blob/f789ab0815b926bd68af6e901cd0e33b2895db3f/dbt/include/sqlserver/macros/adapters/columns.sql#L1-L18

microsoft/dbt-fabric:
https://github.com/microsoft/dbt-fabric/blob/45dacbc87e1b45602dd46218c75f3aaa441dbb95/dbt/include/fabric/macros/adapters/columns.sql#L55-L117

[baldwicc]

In our local projects we've just overridden this by lifting the alter_column_type macro from dbt-msft/dbt-sqlserver as we update to v1.7.x, as Synapse does support ALTER TABLE and sp_rename for columns:

{# baldwicc 2024-05-24 - fabric adapter doesn't support ALTER TABLE statements, but synapse does... and so did sqlserver... #}

{% macro alter_column_type(relation, column_name, new_column_type) %}

  {%- set tmp_column = column_name + "__dbt_alter" -%}

  {% call statement('alter_column_type') -%}
    alter {{ relation.type }} {{ relation }} add "{{ tmp_column }}" {{ new_column_type }};
  {%- endcall -%}
  {% call statement('alter_column_type') -%}
    update {{ relation }} set "{{ tmp_column }}" = "{{ column_name }}";
  {%- endcall -%}
  {% call statement('alter_column_type') -%}
    alter {{ relation.type }} {{ relation }} drop column "{{ column_name }}";
  {%- endcall -%}
  {% call statement('alter_column_type') -%}
    exec sp_rename @objname = '{{ relation(database=False) | replace('"', '') }}.{{ tmp_column }}', @newname = '{{ column_name }}', @objtype = 'column'
  {%- endcall -%}
  
{% endmacro %}