From the field:
Currently we want a key on external_student_test of:
Sounds like we need to add a discriminator to that. Thinking it could be a single-char column, defaulted to '1'.
The changeset/s would need to split up the introduction of that column and the re-creation of the key so implementors have the opportunity to update their datasets. I.e. restart with natural keys disabled, get the new column, fix that column via updates to external data feeds, restart with natural key/s enabled.