Add values to array object in json file - json

Question

I need to add an array of string to a json file.
i.e let's say i have the following json file:
{
"user": {
"role": "admin",
"email": "admin#domain.com"
},
"region": [
{
"location": "EU",
"currency": "EUR",
"countries": [
{
"FR": [
{
"time": "morning",
"cities": []
}
]
}
]
},
{
"location": "NA",
"currency": "USD",
"countries": [
{
"USA": [
{
"time": "evening",
"cities": []
}
]
}
]
}
]
}
Now lets say I have a list of cities that I want to add to region -> countries -> FR
This is my approach, but not sure how to update the cities field.
I have commented the section in the code that need to store to update the list of cities for FA.
func test (jsonFilePath, cityNames string) error {
jsonBytes, err := ioutil.ReadFile(jsonFile)
if err != nil {
return err
}
fmt.Printf("template json file is %s\n", jsonBytes)
var result map[string]interface{}
err = json.Unmarshal(jsonBytes, &result)
if err != nil {
return err
}
if _, exists := result["region"]; exists {
fmt.Printf("Found region\n")
countries := result["region"].([]interface{})
for _, country := range countries {
m := country.(map[string]interface{})
if _, exists := m["FR"]; exists {
/*How to add cityNames to the cities array of the FA?*/
}
}
} else {
errMsg := "region field is missing"
return errors.New(errMsg)
}
jsonBytes, err = json.Marshal(result)
if err != nil {
return err
}
fmt.Printf("the new json file is %s\n", string(jsonBytes))
// Write back to file
err = ioutil.WriteFile("newJson.json", jsonBytes, 0644)
return err
}
Note: The json file has other fields which should not be updated, only Cities filed should be updated; so the preference is to update the Cities field only without defining a struct

Answer 1

Normally it's better (i.e. easier and more efficient) to use structs instead of reflection. There are even websites that can generate a struct from a JSON sample.
But here's how to do it using maps of interfaces:
var result map[string]interface{}
err := json.Unmarshal(jsonBytes, &result)
if err != nil {
return nil, err
}
regions, ok := result["region"].([]interface{})
if !ok {
return nil, errors.New("region not found")
}
for i := range regions {
region, ok := regions[i].(map[string]interface{})
if !ok {
continue
}
countries, ok := region["countries"].([]interface{})
if !ok {
continue
}
for j := range countries {
country, ok := countries[j].(map[string]interface{})
if !ok {
continue
}
fr, ok := country["FR"].([]interface{})
if !ok {
continue
}
for k := range fr {
time, ok := fr[k].(map[string]interface{})
if !ok {
continue
}
cities, ok := time["cities"].([]interface{})
if !ok {
continue
}
time["cities"] = append(cities, cityNames)
}
}
}
Note that in Go maps do not preserve the order, so the order of fields in the output will be different (and also random between runs).