When serializing to JSON, how to omit certain fields based on a run-time condition?

In a web service implemented in Go, I want to be able to restrict fields returned in a JSON response based on a user's role.

For example I may have a currently logged in user who has a role of guest and another with the role of admin

For an admin I want json to have all the keys eg

{
  id: 1,
  name: "John",
  role: "admin"
}

and for a guest to not have the role key eg

{
  id: 1,
  name: "John"
}

I can currently marshal the json and it returns all fields. I need to be able to restrict it.

You can go by the suggestion @Volker made and clear struct fields for which the user has no permissions. This is probably the easiest to implement.

A second option in a similar vein is to create a custom JSON encoder. One which encodes fields only if a role struct tag matches the current user's role. Here is some pseudo code to illustrate:

type T struct {
	currentRole Role   `json:"-"`
	FieldA      string `json:"field_a,omitempty", role:"guest"`
	FieldB      string `json:"field_b,omitempty", role:"guest"`
	FieldC      int    `json:"field_c,omitempty", role:"admin"`
}

// Have T implement the encoding/json.Marshaler interface.
func (t *T) MarshalJSON() ([]byte, error) {
	var buf bytes.Buffer
	
	// Use some reflection magic to iterate over struct fields.
	for _, field := range getStructFields(t) {
		// More reflection magic to extract field tag data.
		role := getFieldTag(field, "role")
		
		// If the field tag's role matches our current role,
		// we are good to go. otherwise, skip this field.
		if !matchingRole(role, t.currentRole) {
			continue // skip this field 
		}

		data, err := json.Marshal(fieldValue(field))
		...
		_, err = buf.Write(data)
		...
	}
	
	return buf.Bytes(), nil
}

This is going to be a pain to maintain if you need new roles though. So this would not be something I would lightly consider doing.

Security concerns

I am not entirely sure that what you are looking for is the right solution to your problem. This depends on the context in which you use your code, which is not clear from your question. But if this concerns a website where a user's abilities on the website are defined solely by the value of the role JSON field, then you are looking at a security hole. They can simply go into a browser debugger and change the value of this JSON object to include the "role: "admin" field. And presto! Instant administrative powers. Whether or not to render certain parts of a page, based on user role, should really be handled by the server, during template processing. Just like any and all data posted to the server should be checked and checked again to ensure it came from a trusted source.

If none of this is applicable to you, then by all means, disregard this paragraph.

This question seems old, but I recently wanted to do the same thing. Maybe this will help someone in the future. Here is another method: you can define your own Marshal interface and use anonymous structs.

//User holds all variables
//even private ones
type User struct {
	ID      int64
    Name    string
    Role    string
}

//MarshalJSON gives back json user
//but only the public fields!
func (u *User) MarshalJSON() ([]byte, error) {
	return json.Marshal(struct {
		ID string `json:"id"`
		Name string `json:"name"`
	}{u.ID, u.Name})
}

it would be pretty easy to put an if u.Role == "admin" statement in the block to decide whether to marshal the rest.

Another option that also works to define the set of fields in the output for a list of struct that comes from an appengine datastore query.

    // Setting different JSON output field for the same struct, using anonymous
    // fields (inspired by inspired by http://choly.ca/post/go-json-marshalling/)

    // This alternative could be used to load a resultset from an appengine datastore
    // query and returned a custom field combination for the list items.

    package main

    import (
    	"encoding/json"
    	"fmt"
    )

    type User struct {
    	ID         string `json:"id"`
    	Name       string `json:"name"`
    	Role       string `json:"-"`
    	LaunchCode string `json:"-"`
    }

    type AdminOutputUser User

    func (user *AdminOutputUser) MarshalJSON() ([]byte, error) {
    	type Alias AdminOutputUser
    	return json.Marshal(&struct {
    		*Alias
    		Role string `json:"role"`
    	}{
    		(*Alias)(user),
    		user.Role,
    	})
    }

    type SuperadminOutputUser User

    func (user *SuperadminOutputUser) MarshalJSON() ([]byte, error) {
    	type Alias SuperadminOutputUser
    	return json.Marshal(&struct {
    		*Alias
    		Role       string `json:"role"`
    		LaunchCode string `json:"code"`
    	}{
    		(*Alias)(user),
    		user.Role,
    		user.LaunchCode,
    	})
    }

    func main() {
    	user := User{"007", "James Bond", "admin", "12345678"}
    	adminOutput := AdminOutputUser(user)
    	superadminOutput := SuperadminOutputUser(user)

    	b, _ := json.Marshal(&user)
    	fmt.Printf("%s\n\n", string(b))
    	// {"id":"007","name":"James Bond"}

    	b, _ = json.Marshal(&adminOutput)
    	fmt.Printf("%s\n\n", string(b))
    	// {"id":"007","name":"James Bond","role":"admin"}

    	b, _ = json.Marshal(&superadminOutput)
    	fmt.Printf("%s\n\n", string(b))
    	// {"id":"007","name":"James Bond","role":"admin","code":"12345678"}
    }

    // for appengine could do something like
    // ...
    // var users []AdminOutputUser // or User or SuperadminOutputUser
    // q := datastore.NewQuery("User")
    // keys, err := q.GetAll(ctx, &users)
    // ...

https://play.golang.org/p/ignIz0hP0z

You might just define your struct like this

type User struct {
	ID   int64 `json:"id"`
	Name string `json:"name"`
	Role string `json:"role,omitempty"`
}

And then set them like this

normalUser := User{ID: "boring", Name: "Rubber"}
adminUser := User{ID: "powers", Name: "Ruler", Role: "admin"}

Then json.Marshal() or json.NewEncoder().Encode() as usual

Found in How To Use Struct Tags in Go

> Note: I know that omitempty was mentioned in a comment and is even
> part of @jimt's code example and mentioned as first option, albeit without a simple example. To me being still pretty new to
> Go wasn't clear that that would just work as expected. So I figured it might help others as well 🤓