[CT-1829] [Bug] [v1.4.0rc1] Back compat in get_merge_sql macro signature

[jtcohen6]

Prompted by Slack thread

Context

In #5702, we changed the signature of the get_merge_sql() macro. This could have implications for users who call get_merge_sql() directly from a custom macro/materialization.

In main / 1.4.latest:

dbt-core/core/dbt/include/global_project/macros/materializations/models/incremental/merge.sql

Lines 1 to 5 in 065ab2e

	{% macro get_merge_sql(target, source, unique_key, dest_columns, incremental_predicates) -%}
	{{ adapter.dispatch('get_merge_sql', 'dbt')(target, source, unique_key, dest_columns, incremental_predicates) }}
	{%- endmacro %}
	{% macro default__get_merge_sql(target, source, unique_key, dest_columns, incremental_predicates) -%}

In 1.3.latest:

dbt-core/core/dbt/include/global_project/macros/materializations/models/incremental/merge.sql

Lines 1 to 5 in 3f17044

	{% macro get_merge_sql(target, source, unique_key, dest_columns, predicates=none) -%}
	{{ adapter.dispatch('get_merge_sql', 'dbt')(target, source, unique_key, dest_columns, predicates) }}
	{%- endmacro %}
	{% macro default__get_merge_sql(target, source, unique_key, dest_columns, predicates) -%}

Repro case

This means that if someone has custom code like the following, they'll start seeing a compilation error in v1.4. (Yes, these are very silly examples! No one should ever call this macro like this.)

-- missing 5th position arg
{{ get_merge_sql(this, this, 'id', ['a', 'b']) }}

Compilation Error in model some_model (models/some_model.sql)
  parameter 'incremental_predicates' was not provided

  > in macro get_merge_sql (macros/materializations/models/incremental/merge.sql)
  > called by model some_model (models/some_model.sql)

-- using old kwarg name
{{ get_merge_sql(this, this, 'id', ['a', 'b'], predicates=['blabla']) }}

Compilation Error in model some_model (models/some_model.sql)
  macro 'dbt_macro__get_merge_sql' takes no keyword argument 'predicates'

Resolution

I think we can provide backwards compatibility quite simply by:

• switching incremental_predicates back to optional, by providing a default value of None
• detecting if predicates is passed as a kwarg, and passing it into incremental_predicates if so

 {% macro get_merge_sql(target, source, unique_key, dest_columns, incremental_predicates=None) -%} 
   -- back compat for old kwarg name
   {% set incremental_predicates = kwargs.get('predicates', incremental_predicates) %}
   {{ adapter.dispatch('get_merge_sql', 'dbt')(target, source, unique_key, dest_columns, incremental_predicates) }} 
 {%- endmacro %} 

With that change, both of the silly code examples above start working again.

(If someone passes both predicates and incremental_predicates as keyword arguments, that's on them.)