Retrieve element from nested JSON string - json

Question

This is the Go code that I have:
func main(){
s := string(`{"Id": "ABC123",
"Name": "Hello",
"RelatedItems":[
{"RId":"TEST123","RName":"TEST1","RChildren":"Ch1"},
{"RId":"TEST234","RName":"TEST2","RChildren":"Ch2"}]
}`)
var result map[string]interface{}
json.Unmarshal([]byte(s), &result)
fmt.Println("Id:", result["Id"])
Rlist := result["RelatedItems"].([]map[string]interface{})
for key, pist := range pist {
fmt.Println("Key: ", key)
fmt.Println("RID:", pist["RId"])
}
}
The struct is down below
type Model struct {
Id string `json:"Id"`
Name string `json:"ModelName"`
RelatedItems []RelatedItems `json:"RelatedItems"`
}
type RelatedItems struct {
RId string `json:"PCId"`
RName string `json:"PCName"`
RChildren string `json:"string"`
}
How would I get an output that would let me choose a particular field from the above?
eg:
Output
Id: ABC123
key:0
RID:TEST123
key:1
RID:TEST234
I am seeing this error
panic: interface conversion: interface {} is nil, not []map[string]interface {}

Answer 1

Based on the posted content,
I'm clear that you are facing issues retrieving data from the nested JSON string.
I've taken your piece of code and tried to compile and reproduce the issue.
After observing, I have a few suggestions based on the way the code has been written.
When the datatype present in the s is known to be similar to the type Model, the result could have been declared as type Model.
That results in var result Model instead of map[string]interface{}.
When the data that's gonna be decoded from the interface{} is not known, the usage of switch would come into rescue without crashing the code.
Something similar to:
switch dataType := result["RelatedItems"].(type){
case interface{}:
// Handle interface{}
case []map[string]interface{}:
// Handle []map[string]interface{}
default:
fmt.Println("Unexpected-Datatype", dataType)
// Handle Accordingly
When we try to Unmarshal, we make sure to look into the json tags that are provided for the fields of a structure. If the data encoded is not having the tags we provided, the data will not be decoded accordingly.
Hence, the result of decoding the data from s into result would result in {ABC123 [{ } { }]} as the tags of the fields Name, RId, RName, RChildren are given as ModelName, PCId, PCName, string respectively.
By the above suggestions and refining the tags given, the piece of code would be as following which would definitely retrieve data from nested JSON structures.
s := string(`{"Id": "ABC123",
"Name": "Hello",
"RelatedItems":[
{"RId":"TEST123","RName":"TEST1","RChildren":"Ch1"},
{"RId":"TEST234","RName":"TEST2","RChildren":"Ch2"}]
}`)
var result Model
json.Unmarshal([]byte(s), &result)
fmt.Println(result)
type Model struct {
Id string `json:"Id"`
Name string `json:"Name"`
RelatedItems []RelatedItems `json:"RelatedItems"`
}
type RelatedItems struct {
RId string `json:"RId"`
RName string `json:"RName"`
RChildren string `json:"RChildren"`
}
This results in the output: {ABC123 Hello [{TEST123 TEST1 Ch1} {TEST234 TEST2 Ch2}]}

Answer 2

Why would you unmarshal to a map anyway and go through type checks?
type Model struct {
Id string `json:"Id"`
Name string `json:"ModelName"`
RelatedItems []RelatedItems `json:"RelatedItems"`
}
type RelatedItems struct {
RId string `json:"PCId"`
RName string `json:"PCName"`
RChildren string `json:"string"`
}
s := `{"Id": "ABC123",
"Name": "Hello",
"RelatedItems":[
{"RId":"TEST123","RName":"TEST1","RChildren":"Ch1"},
{"RId":"TEST234","RName":"TEST2","RChildren":"Ch2"}]
}`
var result Model
if err := json.Unmarshal([]byte(s), &result); err != nil {
log.Fatal(err.Error())
}
fmt.Println("Id: ", result.Id)
for index, ri := range result.RelatedItems {
fmt.Printf("Key: %d\n", index)
fmt.Printf("RID: %s\n", ri.RId)
}