code: def _validate_roles(model):
      
      required_roles = (ATTRIBUTED_ROLE_KEYS[0], ATTRIBUTED_ROLE_KEYS[4],)
      for role_key in ATTRIBUTED_ROLE_KEYS:
          try:
              roles = model.metadata[role_key]
          except KeyError:
              if role_key in required_roles:
                  raise exceptions.MissingRequiredMetadata(role_key)
          else:
              if role_key in required_roles and len(roles) == 0:
                  raise exceptions.MissingRequiredMetadata(role_key)
          for role in roles:
              if role.get('type') != 'cnx-id':
                  raise exceptions.InvalidRole(role_key, role)
docs: Given the model, check that all the metadata role values
      have valid information in them and any required metadata fields
      contain values.