code: def add_subject(str, type = "12")
        subject = ::ONIX::Subject.new
        subject.subject_scheme_id = type.to_i
        subject.subject_code = str
        product.subjects << subject
      end
docs: add a new subject to this product
   str should be the subject code
   type should be the code for the subject scheme you're using. See ONIX codelist 27.
   12 is BIC