Map of Struct from json in Golang

Question

So I am trying to parse a json into some structs and that works ok with the following:

type train struct {
 ID     string  `json:"id"`
 Price  float64 `json:"price,string"`
 Distance float64 `json:"Distance,string"`
}

type Station struct {
 ID       int64  `json:"id,string"`
 arrTrain []train`json:"arr"`
 depTrain []train`json:"dep"`
}

The problem, however, is that I would like to easily be able to reference the items in arrTrain and depTrain using their ID, so I think I need to change the Station struct to have arrTrain and depTrain as maps with the ID as the key. Is this possible when parsing the json or does it have to be 'post-processed'?

EDIT: As stated in one of the comments, unfortunately my json is in the following form:

{
  "id":1, 
  "arr": [
     {"id":"one","price":"$10.1","Distance":"100km"},
     {...}
  ],
  "dep":[
    {"id":"one","price":"$10.1","Distance":"100km"},
    {...}
  ]
}

In other words the ID is not on the outside of the json object and arrTrain is list.

I tried some of the answers below and it seems that maps formed within structs are not recognized/lost by json.Marshal - playground — Conner
– Conner, Commented Mar 16, 2017 at 1:35

netdigger · Accepted Answer · 2017-03-15 21:44:49Z

2

Yes, you can define station like this:

type Station struct {
    ID       int64            `json:"id,string"`
    arrTrain map[string]train `json:"arr"`
    depTrain map[string]train `json:"dep"`
}

And your JSON should like this

{
  "id":1, 
  "arr": {
    "one":{"id":"one","price":"$10.1","Distance":"100km"},
    "two":...
  },
  "dep":{
    "one":{"id":"one","price":"$10.1","Distance":"100km"},
    "two":...
  }
}

answered Mar 15, 2017 at 21:44

netdigger

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3 Comments

RexFuzzle Over a year ago

Ah, I see, thank you. Unfortunately, my json has the form "arr":[{},{}], i.e. the key is not outside the individual item.

netdigger Over a year ago

@RexFuzzle Base on this information, I think the best way is defining UnmarshalJSON function for Station struct.

Benjamin Kadish Over a year ago

@netdigger see my answer but make the field names capitalized.

Benjamin Kadish · Accepted Answer · 2017-03-16 11:59:43Z

1

Edit: Yes you can. (an earlier version of this answer stated keys can only be strings, but as of 1.7 this is not true)

You can see this in action with this playground

Also as conner points out in the comments your field names will have to be exportable for encoding/json to work

type Station struct {
    ID       int64            `json:"id,string"`
    ArrTrain map[string]train `json:"arr"`
    DepTrain map[string]train `json:"dep"`
}

edited Mar 16, 2017 at 11:59

answered Mar 15, 2017 at 21:18

Benjamin Kadish

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1 Comment

mkopriva Over a year ago

since 1.7 encoding/json supports map keys with integer types or keys that implement the encoding.TextMarhsaler/TextUnmarshaler interface. release notes, commit

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