only one subnet_id is allowed behind a router for vpnservice object

I think we should allow more than subnet_id in one vpnservice object.
but the model below limits only one subnet_id is used.
https://github.com/openstack/neutron/blob/master/neutron/extensions/vpnaas.py
RESOURCE_ATTRIBUTE_MAP = {

    'vpnservices': {
        'id': {'allow_post': False, 'allow_put': False,
               'validate': {'type:uuid': None},
               'is_visible': True,
               'primary_key': True},
        'tenant_id': {'allow_post': True, 'allow_put': False,
                      'validate': {'type:string': None},
                      'required_by_policy': True,
                      'is_visible': True},
        'name': {'allow_post': True, 'allow_put': True,
                 'validate': {'type:string': None},
                 'is_visible': True, 'default': ''},
        'description': {'allow_post': True, 'allow_put': True,
                        'validate': {'type:string': None},
                        'is_visible': True, 'default': ''},
        'subnet_id': {'allow_post': True, 'allow_put': False,
                      'validate': {'type:uuid': None},
                      'is_visible': True},
        'router_id': {'allow_post': True, 'allow_put': False,
                      'validate': {'type:uuid': None},
                      'is_visible': True},
        'admin_state_up': {'allow_post': True, 'allow_put': True,
                           'default': True,
                           'convert_to': attr.convert_to_boolean,
                           'is_visible': True},
        'status': {'allow_post': False, 'allow_put': False,
                   'is_visible': True}
    },

with such limit, I don't think there is a way to allow other subnets behind the router be vpn exposed!