isse while accessing synthetic attributes within resource group in heat

The question is related to openstack heat templates.

I am not sure about what's the wrong with the below situation.

I have a resource group that creates multiple neutron Ports.

While trying to use the output of this resource group, i am trying to use the synthetic attributes but it is just impossible. the error was that the attribut is not there actually. and by showing the strcture of the resource effectively i cannot see the attribut resource. here after the code to clarify more.

The definition of my resource group.

  bckg1_ports_group:
    depends_on:
      - datag1_subnet
    type: OS::Heat::ResourceGroup
    condition: create_g1
    properties:
      count: { get_param: bck_nbrg1 }
      resource_def:
        type: OS::Neutron::Port
        properties:
          name: 
            list_join: ['', ['backg1vm', '%index%', '_data_port']]
          network: { get_resource: "datag1_network" }
          fixed_ips: [{"subnet": { get_resource: "datag1_subnet" }, "ip_address": 192.168.10.3%index%}]
          allowed_address_pairs: [{"ip_address": "0.0.0.0/0"}]
          security_groups: [{ get_resource: open_secgroup }]

To access the attributes of the resources inside the resource group i use.

   portgroup_rsc:
     condition: create_g1
     description: G1 group RSC
     value: { get_attr: [bckg1_ports_group, resource] }

   portgroup_rsc0:
     condition: create_g1
     description: G1 group RSC
     value: { get_attr: [bckg1_ports_group, resource.0] }

   portgroup_rsc0_mac:
     condition: create_g1
     description: G1 group RSC
     value: { get_attr: [bckmg1_ports_group, resource.0.mac_address] }

None of these works because the upper attribute resource is not there. i can use refs or refs_map but only to get IDs.

the error.

ERROR: resources.bckg1_ports_group<nested_stack>.outputs.resource.Value[0].get_attr: : The Referenced Attribute (0 resource) is incorrect.