How to handle missing fields in a JSON response dynamically in Go - json

Question

I'm working on a Go wrapper for an API and I noticed that two of the JSON fields stay empty when they don't have any data.
Basically the API returns a set of information on a given url, and if it was visited at least once, everything is okay and I get a full json that I then Unmarshal into a struct:
{
"stats":{
"status":1,
"date":"09.07.2019",
"title":"Test",
"devices":{
"dev":[
{
"tag":"Desktop"
}
],
"sys":[
{
"tag":"GNU/Linux "
},
{
"tag":"Windows 10"
}
],
"bro":[
{
"tag":"Firefox 67.0"
},
{
"tag":"Chrome 62.0"
}
]
},
"refs":[
{
"link":"www.google.com"
}
]
}
}
This is the struct I'm using:
type Stats struct {
Stats struct {
Status int `json:"status"`
Date string `json:"date"`
Title string `json:"title"`
Devices struct {
Dev []struct {
Tag string `json:"tag"`
} `json:"dev"`
Sys []struct {
Tag string `json:"tag"`
} `json:"sys"`
Bro []struct {
Tag string `json:"tag"`
} `json:"bro"`
} `json:"devices"`
Refs []struct {
Link string `json:"link"`
} `json:"refs"`
} `json:"stats"`
}
When a new url is given, then things become a little bit weird:
{
"stats": {
"status": 1,
"date": "09.07.2019",
"title": "Test2",
"devices": [
],
"refs": [
]
}
}
As you can see, the fields "dev", "sys" and "bro" just disappear because they're not used and when I try to Unmarshal the JSON into the same struct I get json: cannot unmarshal array into Go struct field Stats.device of type [...]
I tried to use two different structs to handle both the responses but I'm sure that there's a way to handle them gracefully with just one.
Any help would be appreciated, thanks!

Answer 1

I finally managed to make it work with an ugly workaround.
I changed my struct to
type Stats struct {
Status int `json:"status"`
Date string `json:"date"`
Title string `json:"title"`
Devices interface{} `json:"devices"`
Refs interface{} `json:"refs"`
}
Then I can finally Unmarshal the JSON in both cases, but I get a map[string]interface{} when an object is passed and an empty interface{} when an empty array is passed. In order to fix this inconsistency, I simply check for the data type and force the use of a JSON intermediate conversion in order to unpack the map[string]interface{} value inside a custom Devices struct:
// Devices contains devices information
type Devices struct {
Dev []struct {
Tag string `json:"tag"`
Clicks string `json:"clicks"`
} `json:"dev"`
Sys []struct {
Tag string `json:"tag"`
Clicks string `json:"clicks"`
} `json:"sys"`
Bro []struct {
Tag string `json:"tag"`
Clicks string `json:"clicks"`
} `json:"bro"`
}
The algorithms I use are the following:
//ForceDevicesToRightType uses a json conversion as intermediary for filling the Stats.Devices
// struct with map[string]interface{} values
func ForceDevicesToRightType(dev interface{}) (Devices, error) {
temp, err := json.Marshal(dev)
if err != nil {
return Devices{}, err
}
// Use a temporary variable of the right type
var devices Devices
err = json.Unmarshal(temp, &devices)
if err != nil {
return Devices{}, err
}
return devices, nil
}
// ForceRefsToRightType uses a json conversion as intermediary for filling the Stats.Refs
// struct with map[string]interface{} values
func ForceRefsToRightType(refs interface{}) (Refs, error) {
temp, err := json.Marshal(refs)
if err != nil {
return Refs{}, err
}
// Use a temporary variable of the right type
var references Refs
err = json.Unmarshal(temp, &references)
if err != nil {
return Refs{}, err
}
return references, nil
}
Since the compiler knows that both Devices and Refs fields are interface{} I cannot simply access any methods after the conversion, so I simply make a cast of the right type and everything works fine.
For example, if I wanted to access the Dev sub-struct, this is the proper way:
y, _ := GetStats()
fmt.Println(y.Devices.(Devices).Dev)
It's ugly, but it works.
Thank you very much for your help, I hope that this method will save you an headache!