Flow Builder Transform Element: ID Field Causes Validation Error When Selected as Join Key

This article answers the question: why does selecting the Id field, or any other field, as a Join Key in the Flow Builder Transform element cause a validation error, and how do you fix it? The Transform element in Flow Builder supports joining two collections using an Inner Join. It requires three mandatory steps to be completed:

1. Configure the join keys for each source collection
2. Select the join fields to return into the target collection
3. Map the fields from the joined source collections to the target collection

The validation errors above appear when step 2 (selecting join fields) or step 3 (mapping fields) is skipped or left incomplete — the error isn't specific to the Id field itself, but to leaving the join configuration incomplete after choosing a join key.

For detailed configuration steps, refer to Join Collections in a Flow.