logstash 如何处理 mongodb 导出来的 _id value数据。 how to custom fields of logstash by mongo mapreduce exported data.(example format: {_id:"xxx"} , value:{})

input {  
      file {
          path => "c:\aa.json"
          start_position => "beginning"
          #sincedb_path => "NUL"
          codec => json {
            charset => "UTF-8"
        }       
      }
}
filter {  
   json{
   source => "message"
   }
    #custom fields
    ruby {
       code => "event.set('users',event.get('value')['usertag'])
                event.set('logcreatetime',event.get('value')['createtime'])
                event.set('usertype',event.get('value')['usertype'])
                event.set('userid',event.get('value')['userid'])
       "
    }

    mutate
    {
        remove_field => [ "message","path","@version","@timestamp","host","_id","value"] #删除字段 delete fields
    }
}
output {  
    elasticsearch {
        action => "index"
        hosts => ["http://localhost:9200"]
        index => "urnesteddb"
        document_type => "usertable"
        workers => 1
    }
     #stdout {
         #codec => rubydebug
         #codec => json_lines
     #}
}