Missing nested array of struct object after json unmarshaling

Question

I ran into some problem with unmarshalling string back to struct object when the object has a nested array of struct objects. I use the following code as a demo for my problem:

The json string is

const myStr = `{
  "name": "test_session1",
  "shared_items:": [
    {
      "id": 0,
      "name": ".aspnet"
    },
    {
      "id": 1,
      "name": ".bash_profile"
    }
  ]
}`

I have two structs as following with Session being the parent and SharedItem being the associated child(ren):

type Session struct {
    ID              uint64           `json:"id"`
    Name            string           `json:"name,omitempty"`
    SharedItems     []SharedItem `json:"shared_items"`
}

type SharedItem struct {
    ID          uint64 `json:"id"`
    Name        string `json:"name"`
}

I tried the following to unmarshal the json string, but it looks like the nested array of SharedItem objects are missing, as I saw sess object has 0 shared items, which is not what I expected.

func main() {
    var sess Session
    if err := json.Unmarshal([]byte(myStr), &sess); err != nil {
        panic(err)
    }
    fmt.Printf("sess name is: %s, and has %d shared items\n", sess.Name, len(sess.SharedItems)) // printed: sess name is test_session1, and has 0 shared items
}

Here's the link to my go playground: https://play.golang.org/p/a-y5T3tut6g

Himanshu · Accepted Answer · 2018-04-26 05:57:23Z

3

Golang Spec for json unmarshal describe it all

To unmarshal JSON into a struct, Unmarshal matches incoming object keys to the keys used by Marshal (either the struct field name or its tag), preferring an exact match but also accepting a case-insensitive match. By default, object keys which don't have a corresponding struct field are ignored (see Decoder.DisallowUnknownFields for an alternative).

In the json provided the json object for shared items has : colon in it as we can see it is shared_items: not shared_items which is our json tag

  "shared_items:": [
    {
      "id": 0,
      "name": ".aspnet"
    },
    {
      "id": 1,
      "name": ".bash_profile"
    }
  ]

Either remove : or append into your struct json tag to match the same.

package main

import (
    "fmt"
    "encoding/json"
)

type Session struct {
    Name            string           `json:"name,omitempty"`
    SharedItems     []SharedItem    `json:"shared_items"`
}

type SharedItem struct {
    ID          uint64 `json:"id"`
    Name        string `json:"name"`
}

const myStr = `{"name":"test_session1","shared_items":[{"id":0,"name":".aspnet"},{"id":1,"name":".bash_profile"}]}`

func main() {
    var sess Session

    if err := json.Unmarshal([]byte(myStr), &sess); err != nil {
        panic(err)
    }
    fmt.Println(sess)
    fmt.Printf("sess name is: %s, and has %d shared items\n", sess.Name, len(sess.SharedItems))
}

Playground example

edited Apr 26, 2018 at 5:57

answered Apr 26, 2018 at 5:51

Himanshu

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2 Comments

TonyW Over a year ago

thanks, it turned to be a typo with the colon accidentally added after shared_items.

Himanshu Over a year ago

for unmarshalling json struct tags should match exactly with the json objects

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