Logstash 5.0 Ruby Filter Can't Update Hash in Array

[Java_Beta]

I'm a newbie to Logstash and Ruby, and I meet a subtle problem today.

My input JSON like the following:

{
  "1": "1",
  "2": "2",
  "market": [
    {
      "id": "1",
      "name": "m1"
    },
    {
      "id": "2",
      "name": "m2"
    }
  ]
}

My filter is:

filter {
    ......
    ruby {
        code => "
        markets = event.get('market')
        markets.each_index do |index| 
    event.set(markets[index]['id'], markets[index]['name']) 
    markets[index]['id'] = markets[index]['name']
    end
"
}
......
}

And the output is:

{
  "1": "m1",
  "2": "m2",
  "market": [
    {
      "id": "1",
      "name": "m1"
    },
    {
      "id": "2",
      "name": "m2"
    }
  ]
}

What I expect to be:

{
  "1": "m1",
  "2": "m2",
  "market": [
    {
      "id": "m1",
      "name": "m1"
    },
    {
      "id": "m2",
      "name": "m2"
    }
  ]
}

I think market.id should be m1 not 1, could anyone give me some clue what's wrong with my filter?
The logstash I'm using is version 5.0.