Golang issue with accessing Nested JSON Array after Unmarshalling

Question

I'm still in the learning process of Go but am hitting a wall when it comes to JSON response arrays. Whenever I try to access a nested element of the "objects" array, Go throws (type interface {} does not support indexing)

What is going wrong and how can I avoid making this mistake in the future?

package main    

import (
        "encoding/json"
        "fmt"
)    

func main() {
        payload := []byte(`{"query": "QEACOR139GID","count": 1,"objects": [{"ITEM_ID": "QEACOR139GID","PROD_CLASS_ID": "BMXCPGRIPS","AVAILABLE": 19}]}`)
        var result map[string]interface{}
        if err := json.Unmarshal(payload, &result); err != nil {
            panic(err)
        }        

        fmt.Println(result["objects"]["ITEM_ID"])    

}

http://play.golang.org/p/duW-meEABJ

edit: Fixed link

@fabrizioM, the example at play.golang.org is not the same as the code listed in the question. — sberry
– sberry, Commented Jun 24, 2014 at 4:01

James Henstridge · Accepted Answer · 2014-06-24 03:46:23Z

28

As the error says, interface variables do not support indexing. You will need to use a type assertion to convert to the underlying type.

When decoding into an interface{} variable, the JSON module represents arrays as []interface{} slices and dictionaries as map[string]interface{} maps.

Without error checking, you could dig down into this JSON with something like:

objects := result["objects"].([]interface{})
first := objects[0].(map[string]interface{})
fmt.Println(first["ITEM_ID"])

These type assertions will panic if the types do not match. You can use the two-return form, you can check for this error. For example:

objects, ok := result["objects"].([]interface{})
if !ok {
    // Handle error here
}

If the JSON follows a known format though, a better solution would be to decode into a structure. Given the data in your example, the following might do:

type Result struct {
    Query   string `json:"query"`
    Count   int    `json:"count"`
    Objects []struct {
        ItemId      string `json:"ITEM_ID"`
        ProdClassId string `json:"PROD_CLASS_ID"`
        Available   int    `json:"AVAILABLE"`
    } `json:"objects"`
}

If you decode into this type, you can access the item ID as result.Objects[0].ItemId.

answered Jun 24, 2014 at 3:46

James Henstridge

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4 Comments

sberry Over a year ago

I literally just typed up the same answer. Delayed because I had to carry in the groceries :)

sberry Over a year ago

For future reference, here is a working play example: play.golang.org/p/iYjrB-72u5

John Over a year ago

Thanks for this insight on working with nested JSON structure!

vanarajcs Over a year ago

HI James Henstridge, type Example struct { text []string } type Result struct { Query string Count int Objects []struct { ItemId string } }

StevenR · Accepted Answer · 2018-01-14 13:05:30Z

For who those might looking for similar solution like me, https://github.com/Jeffail/gabs provides better solution.

I provide the example here.

package main

import (
    "encoding/json"
    "fmt"

    "github.com/Jeffail/gabs"
)

func main() {
    payload := []byte(`{
        "query": "QEACOR139GID",
        "count": 1,
        "objects": [{
            "ITEM_ID": "QEACOR139GID",
            "PROD_CLASS_ID": "BMXCPGRIPS",
            "AVAILABLE": 19, 
            "Messages": [ {
                    "first": {
                        "text":  "sth, 1st"
                    }
                },
                {
                        "second": {
                        "text": "sth, 2nd"
                    }
              }
            ]
        }]
    }`)

    fmt.Println("Use gabs:")
    jsonParsed, _ := gabs.ParseJSON(payload)
    data := jsonParsed.Path("objects").Data()
    fmt.Println("  Fetch Data: ")
    fmt.Println("    ", data)
    children, _ := jsonParsed.Path("objects").Children()
    fmt.Println("  Children Array from \"Objects\": ")
    for key, child := range children {
        fmt.Println("    ", key, ": ", child)
        children2, _ := child.Path("Messages").Children()
        fmt.Println("    Children Array from \"Messages\": ")
        for key2, child2 := range children2 {
            fmt.Println("      ", key2, ": ", child2)
        }
    }
}

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