Modifying a changeset with build_assoc?

Howdy, I’m looking for some help with polymorphic associations and nested forms.

I’m using formex (a great form library inspired by Symfony) and formex_ecto which gives a modify_changeset callback. I’m wanting to possibly use it and build_assoc to modify my changeset but I’m not really sure how or if that will even work.

I’ll post code snippets if that would be helpful.