[Rails] Can join tables be polymorphic?

For example:
Currently I have:
functions < functions_competencies < competencies
exams < exam_competencies < competencies

I'd like to dry up "functions_competencies" and "exam_competencies" by
using a polymorphic association, by let's say 'attached_competencies':

functions < attached_competencies < competencies
exams < attached_competencies < competencies
evaluations < attached_competencies < competencies
...

Note that these are join tables.

Is this possible?

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