How to add a meta field to a collection of Eloquent model objects in Laravel

I have a Device model and a Readings model which are connected by a many-to-many relationship. I am working on an API which returns an array of all the registered devices to a particular user. What i want is a field in the response which shows the last entry in the readings table.

This is the method which returns the response.

public function getUserDevices($user_id){

    $devices = Device::where('user_id',$user_id)->get();
    return response()->json($devices);
}

The response that i am currently getting.

[
   {
     "id": 2,
     "user_id": 1,
     "device_name": "My Device",
     "device_type": "Sensor",
     "status": 1,
     "created_at": "2018-05-01 14:39:35",
     "updated_at": "2018-05-13 17:56:56",
     "device_key": "60:01:94:37:D1:34",
     "actuator": 0,
     "mode": 1
   },
   {
     "id": 3,
     "user_id": 1,
     "device_name": "Home Device",
     "device_type": "Sensor",
     "status": 1,
     "created_at": "2018-05-01 14:40:25",
     "updated_at": "2018-05-12 13:42:00",
     "device_key": "60:01:94:37:D1:35",
     "actuator": 0,
     "mode": 0
   }
]

The response that i want.

[
   {
     "id": 2,
     "user_id": 1,
     "device_name": "My Device",
     "device_type": "Sensor",
     "status": 1,
     "created_at": "2018-05-01 14:39:35",
     "updated_at": "2018-05-13 17:56:56",
     "device_key": "60:01:94:37:D1:34",
     "actuator": 0,
     "mode": 1

     "reading":{    <--- This is what i want in every item of the collection
         "temp": 45,
         "hum" : 60
     }

   },
   {
     "id": 3,
     "user_id": 1,
     "device_name": "Home Device",
     "device_type": "Sensor",
     "status": 1,
     "created_at": "2018-05-01 14:40:25",
     "updated_at": "2018-05-12 13:42:00",
     "device_key": "60:01:94:37:D1:35",
     "actuator": 0,
     "mode": 0

     "reading":{    <--- This is what i want in every item of the collection
         "temp": 35,
         "hum" : 76
     }

   }
]

Any help would be appreciated. Thanx

via Chebli Mohamed