code: def _validate_subjects(cursor, model):
      
      subject_vocab = [term[0] for term in acquire_subject_vocabulary(cursor)]
      subjects = model.metadata.get('subjects', [])
      invalid_subjects = [s for s in subjects if s not in subject_vocab]
      if invalid_subjects:
          raise exceptions.InvalidMetadata('subjects', invalid_subjects)
docs: Give a database cursor and model, check the subjects against
      the subject vocabulary.