如何在json文档中向现有数据添加新字段？

I am a beginner in Golang..I have created an API which is reading the data from couchbase well but I am not able to write new fields in the json document..The code of writing new data is below:

func appendDataEndpoint(w http.ResponseWriter,req *http.Request){
    var data map[string]interface{}
    _ = json.NewDecoder(req.Body).Decode(&data)
    fmt.Println(data)
    params := mux.Vars(req)
    str := params["id"]

    message := message{ Student :data["student"].([]struct),//Here is another problem.How to write type of Student since it is referring to another structure."struct" is throwing type error.
                        College :data["college"].(string),
                        CollegeId: data["collegeid"].(string),
                        Hobbies :data["hobbies"].([]string),
                        Firstname: data["firstname"].(string),
                        Address: data["address"].(string),//New field to be inserted
                        Mobile: data["mobile"].(string),//New field to be inserted
    }
    fmt.Println(message.Address)
    _,err:=bucket.Insert(str, message, 0)

    if err!=nil{
        fmt.Println("Error in inserting")
        w.WriteHeader(401)
        w.Write([]byte(err.Error()))
        return
    }
    json.NewEncoder(w).Encode(message)
}

Still having problems.Now the updated code snippet is:

    message:=message{
    Student :data["student"].([]Student),//Error still exists here
    College :data["college"].(string),
    CollegeId :data["collegeid"].(string),
    Hobbies :data["hobbies"].([]string),
    Firstname :data["firstname"].(string),
    Add: data["a"].(string),
    Mo: data["m"].(string),
    }
    _,err:=bucket.Insert(str, message, 0)

The error is:

    panic serving [::1]:63648: interface conversion: interface {} is nil, not []main.Student

A few things to notice since the code you provided is not fully available (what about providing code for the message struct?):

When getting a request from the client, you unmarshal it into a map[string]{}interface object. From there, you construct manually a message object. Why don't you construct the message object without having to work with type assertions? You could try doing something like so:

type Message struct {
    Student []Student `json:"student"`
    College String `json:"college"`
    CollegeId String `json:"collegeid"`
    Hobbies []String `json:"hobbies"`
    FirstName String `json:"firstname"`
    Address String `json:"address"`
    Mobile String `json:"mobile"`
}

type Student struct {
    Name String `json:"name"`
    Qualifications []String `json:"qualifications"`
    Email String `json:"email"`
}

var data Message
if err := json.NewDecoder(req.Body).Decode(&data); err != nil {
    fmt.Println(err)
    return
}

fmt.Println(data)

This would also take care of the student struct you were having difficulties with.

I would recommend reading this article to get you started with JSON and Go.