agree with this - sounds like you just worked on spec for them and built an app their team didn't have time or desire to deal with, and they wanted it to better fit their internal code styles so when they do have to go do maintenance, it's an easier lift. Sorry man!
I would whole heartedly agree this looks very much like free work.
One 1:many relationship is not a production application. Is is clearly a contrived scenario designed not to disadvantage a Dev with no domain knowledge (apart from knowing what an IBAN is...)
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