Change union macro to not always include the _dbt_source_relation column

Describe the feature

When developing, the _dbt_source_relation column from the union macro is useful to check that things are behaving properly, but once it’s all working properly I don’t like exposing implementation details to output models. I’d like to be able to opt out of that column being added at the front of my model.

Describe alternatives you’ve considered

• Adding another intermediary model which uses the star macro, passing the column into the except arg
• Maintaining an internal copy of this macro

Who will this benefit?

Me! In this case, it’s a legacy hard-delete-finding snapshot using my snapshot-sandwich approach, and we’ve added another column but they’re out of order and don’t union nicely anymore.

Are you interested in contributing this feature?

Yes - I was planning to just wrap the cast({{ dbt_utils.string_literal(relation) }} as {{ dbt_utils.type_string() }}) as {{ source_column_name }} line in an {% if source_column_name %} block, so you can pass None in if you don’t want the column. Open to alternative approaches though (a new bool is more explicit, but getting towards a pretty unwieldy signature)

[joellabes]

@chloe-lubin thanks for suggesting this! Could I ask you to open a new issue so that it can be prioritised/discovered separately to this one?

[joellabes]

Resolved by #624