Dbt unique key use multiple clustered columns

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I am new to DBT and trying a unique key containing multiple columns which are clustered in snowflake. I see this article unique_key | dbt Docs

which describes 2 approaches

  1. unique_key=“transaction_id||‘-’||line_item_id”,
  2. unique_key=“id”, where id is concat of two fields

If I go with option 1 it creates issue with merge into statement and that doesn’t seem to understand the prefixes and uses later column without the prefix for table

using 2nd option I am not essentially using clustered columns and the merge will work but is tremendously slow

How can we use unique key with merge and using clustered columns?