lib/onix/apa_product.rb
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
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
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