在运行时覆盖Golang JSON标签值

Question

I have the following struct in in my Web Application using "encoding/json"

type CourseAssignment struct {
    Semester int `json:"semester"  xml:"semester"`
    Lecture Lecture `json:"-"  xml:"-"`
    Cos Cos `json:"-"  xml:"-"`
    Links map[string][]Link `json:"links,omitempty" xml:"links,omitempty"`
}

Lecture and Cos are complex structs themselves that i dont want to be included in my serialized json which i indicate by setting json:"-"

This works perfectly.

I want to override that behavior on demand during runtime, how do i do that without writing my own Serialization code ?

Edit: My Own Solution:

func (r *CourseAssignment) Expand(depth int) CourseAssignment {

    if depth <= 0 {
        return *r
    }

    tmp := *r
    tmp.LectureEx = tmp.Lecture
    tmp.CosEx = tmp.Cos
    tmp.Links = nil 
    return tmp
}

type CourseAssignment struct {
    Semester int `json:"semester"  xml:"semester"`
    Lecture *Lecture `json:"-"  xml:"-"`
    Cos *Cos `json:"-"  xml:"-"`
    Links map[string][]Link `json:"links,omitempty" xml:"links,omitempty"`
    LectureEx  *Lecture   `json:"lecture,omitempty"  xml:"lecture,omitempty"`
    CosEx *Cos `json:"course_of_study,omitempty" xml:"course_of_study,omitempty"`   
}

When i want to include the fields i create a Copy of the Object using expand that fills fields that contain the same references but show up in the serialization.

Answer 1

You can read/get struct tag values using StructTag from reflect package:

package main

import (
    "fmt"
    "reflect"
)

type CourseAssignment struct {
    Semester int `json:"semester"  xml:"semester"`
}

func main() {
    ca := CourseAssignment{}
    st := reflect.TypeOf(ca)
    field := st.Field(0)
    fmt.Println(field.Tag.Get("json"))
}

There is no method to change a struct tag field in the standard library.

However, there are open-source libraries that do exactly that, like Retag.