Have union_relations raise exception when include parameter results in no columns

[grahamwetzler]

This is a:

• bug fix PR with no breaking changes — please ensure the base branch is master
• new functionality — please ensure the base branch is the latest dev/ branch
• a breaking change — please ensure the base branch is the latest dev/ branch

Description & motivation

Fixes #266. Using the include parameter in union_relations with columns names that don't exist in the relations will result in invalid SQL being generated. This PR adds logic to raise an exception if there are no columns to union.

There doesn't appear to be a good way to write a unit test to ensure that an exception is raise (dbt-labs/dbt-core#3060), but this worked in my manual testing.

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
• I followed guidelines to ensure that my changes will work on "non-core" adapters by:
  • dispatching any new macro(s) so non-core adapters can also use them (e.g. the star() source)
  • using the limit_zero() macro in place of the literal string: limit 0
  • using dbt_utils.type_* macros instead of explicit datatypes (e.g. dbt_utils.type_timestamp() instead of TIMESTAMP
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md

[joellabes]

Hi @grahamwetzler, this makes a ton of sense! Thanks for making the change. I think that you're right that there's no way to expect a failure right now. The core team are giving this some thought at the moment though 🤞

One nitpick: could you add the name of the affected model to the error message, to make it easier to track down? For some inspiration, check out

dbt-utils/macros/cross_db_utils/_is_ephemeral.sql

Lines 7 to 12 in 73a6556

	{% set model_name = obj.name %}
	{%- endif -%}
	{% set error_message %}
	The `{{ macro }}` macro cannot be used with ephemeral models, as it relies on the information schema.
	`{{ model_name }}` is an ephemeral model. Consider making it a view or table instead.

(You won't need to pass in the calling macro name)

[grahamwetzler]

The new error message from my testing:

17:43:27  Encountered an error:
Runtime Error
  Compilation Error in model union_test (models/union_test.sql)
    There were no columns found to union for relations u1, u2
    
    > in macro default__union_relations (macros/sql/union.sql)
    > called by macro union_relations (macros/sql/union.sql)
    > called by model union_test (models/union_test.sql)
    > called by model union_test (models/union_test.sql)

[joellabes]

I like it! Could you add yourself to the changelog under the next patch release, and we'll get this merged in? 🎉

[joellabes]

Thank you so much for the contribution! 🥇