code: def handle_relations(changeset):
      
      for relation in changeset.bundle.get('relations', []):
          relations = [models.Relation(*i.split(':')) if ':' in i
                       else models.Relation(i, '') for i in relation]
          changeset.send({
              'id': 'addRelation-{}'.format(changeset.next_action()),
              'method': 'addRelation',
              'args': [
                  '${}'.format(
                      changeset.services_added[rel.name]) +
                  (':{}'.format(rel.interface) if rel.interface else '')
                  for rel in relations
              ],
              'requires': [changeset.services_added[rel.name] for
                           rel in relations],
          })
      return handle_units
docs: Populate the change set with addRelation changes.