Trying to render STI classes gives a cryptic error

[faissalMT]

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Something similar was briefly mentioned here: https://github.com/jsonapi-rb/jsonapi-rails/issues/68#issuecomment-417905177
When a relation contains an item of an STI type jsonapi-rails isn't able to infer the renderer for it and instead it only crashes with the error:

ActiveModel::UnknownAttributeError:
       unknown attribute 'url_helpers' for PrimarySubject.

At the very least a helpful error message would go a long way here, as this is difficult to debug yourself.
The workaround is to manually specify all classes:

{
          Subject: SerializableSubject,
          PrimarySubject: SerializableSubject
          SecondarySubject: SerializableSubject
          ModernLanguagesSubject: SerializableSubject,
          FurtherEducationSubject: SerializableSubject
}