Parsing nested JSON string in Logstash

Question

I am logging to logstash,in json format, my logs have the following fields, each field is a string and the atts field is a stringified json (note: atts sub fields are different each time)

here is an example:

{"name":"bob","last":"builder", "atts":"{\"a\":111, \"b\":222}"}

I would like to parse it to something like this:

   {
     "name" => "bob",
     "last" => "builder"
     "atss" => {
          "a" => 111,
          "b" => 222}
   }

here is my configuration:

input { stdin { } }  

filter {
  json {
    source => "message"
    target => "parsed"
  }
}
 output { stdout { codec => rubydebug  }}

ok, so now I get this:

{
    "@timestamp" => 2017-04-05T12:19:04.090Z,
    "parsed" => {
        "atss" => "{\"a\":111, \"b\":222}",
        "name" => "bob",
        "last" => "the builder"
    },
        "@version" => "1",
          "host" => "0.0.0.0"
}

how can I parse the atts field to json so I receive:

{
    "@timestamp" => 2017-04-05T12:19:04.090Z,
    "parsed" => {
        "atss" => 
           {"a" => 111,
            "b" => 222},
        "name" => "bob",
        "last" => "the builder"
    },
        "@version" => "1",
          "host" => "0.0.0.0"
}

Answer 1

thanks to @Alcanzar here is what I did

input { 
  stdin { } 
}  

filter {
  json {
    source => "message"
    target => "message"
  }
  json {
    source => "[message][atts]"
    target => "[message][atts]"
  }

}
 output { stdout { codec => rubydebug  }}