def download_links
    @downloads = Download.all
    render json: { data:@downloads.as_json(:only=>[:id],
    include: {
        :downloadlink => {}
      }
    ), message:"success"}, status: :ok
  end

{
"data": [
    {
        "id": 1,
        "downloadlink": [
            {
                "url": "1920x1080.png"
            },
            {
                "url": "1366x766.png"
            },
            {
                "url": "1280x720.png"
            },
            {
                "url": "1024x768.png"
            }
        ]
    }
],
"message": "success"}

{
"data": [
    {
        "id": 1,
        "downloadlink": [
                "url": "1920x1080.png",
                "url": "1366x766.png",
                "url": "1280x720.png",
                "url": "1024x768.png"
        ]
    }
],
"message": "success"}