looping over struct and accessing array in golang

Question

My issue is

I want to be able to loop over each server and within that loop, loop over UsageData
My current loop gives me an error about ranging over assets not sure why
I cant access assets.Server.UsageData

Here is my code : https://go.dev/play/p/ttNVW5_Q4Ys

package main

import (
    "encoding/json"
    "fmt"
)

type Asset struct {
    Server struct {
        Host string `json:"host"`
        Port string `json:"port"`
    } `json:"server"`
    Postgres struct {
        Host      string `json:"host"`
        User      string `json:"user"`
        Password  string `json:"password"`
        DB        string `json:"db"`
        UsageData []struct {
            Region string `json:"Region"`
            Mbps   int    `json:"Mpbs"`
        } `json:"UsageData"`
    } `json:"database"`
}

func main() {
    jsonConfig := []byte(`[
    {
        "server":{
            "host":"serverA",
            "port":"8080"},
        "database":{
            "host":"serverA",
            "user":"db_user",
            "password":"supersecret",
            "db":"A_db",
            "UsageData":[{"Region":"US","Mbps":100}, {"Region":"EU","Mbps":140}]
        }
    },
    {
        "server":{
            "host":"serverB",
            "port":"8383"},
        "database":{
            "host":"serverB",
            "user":"db_user2",
            "password":"acbd123",
            "db":"B_db",
            "UsageData":[{"Region":"US","Mbps":58}, {"Region":"EU","Mbps":250}]
        }   
    }
]`)
    var assets []Asset
    err := json.Unmarshal(jsonConfig, &assets)
    if err != nil {
        panic(err)
    }
    fmt.Printf("Assets: %+v\n", assets)
    //fmt.Printf("Config: %+v\n", assets.Server.Host)
    //fmt.Printf("Config: %+v\n", assets.database.UsageData)
    //fmt.Printf("Config: %+v\n", assets.Server.UsageData)

    for _, asset := range assets {
        fmt.Printf("%v\n", asset)
        //for _, Usage := range assets.UsageData {
        //  fmt.Printf("%v\n",Usage)
        //}
    }
}

** Code with the correct answer, I was calling the nested struct incorrectly**

https://go.dev/play/p/tEbA405WWbC

You can't loop over a struct's fields with plain Go, it's not supported by the language. You need to use reflection to be able to do that. You could unmarshal the json into a map which allows looping but that has other drawbacks when compared to struct. — mkopriva, Mar 17 '22 at 08:00
I'm really confused on how to apply reflection. Here's where im stuck now go.dev/play/p/76te17MmJcI — chowpay, Mar 17 '22 at 18:03
you don't need reflection if you are willing to access the fields directly, and to loop over `UserData` which is a slice you also don't need reflection, slices are meant to be iterated over, you just need to correct [selector](https://go.dev/ref/spec#Selectors) to access it: https://go.dev/play/p/cOhUvpUWQPZ — mkopriva, Mar 17 '22 at 18:11

score 2 · Accepted Answer · answered Mar 17 '22 at 11:34

2

Provided jsonConfig is not technically incorrect but keys SHOULD be unique (see question).
It seems that encoding/json will override value with last occurrence.

So you have 2 options:

change jsonConfig to [{asset1}, {asset2}] (see fixed playground)
implement custom unmarshaler

I definitely recommend first option.

answered Mar 17 '22 at 11:34

medasx

634
4
7

Ok I see what you did there with asset1, asset2, oversight on mypart. But how do you loop inside to get to the userData. For example : https://go.dev/play/p/76te17MmJcI – chowpay Mar 17 '22 at 17:59
1

@chowpay You're trying to access field from nested struct. Use `asset.Postgres.UsageData` – medasx Mar 17 '22 at 19:00

looping over struct and accessing array in golang

1 Answers1