One struct with multiple json representations - json

Question

The problem I'm trying to solve is that I have a model of a community that looks like this
type Community struct {
Name string
Description string
Sources []Source
Popularity int
FavoriteCount int
Moderators []string
Children []Community
Tracks []Track
}
Communities hold a lot of information and there are scenarios when I want to return only part of the description such as if I'm returning a list of trending communities. In this case I'd want to return only
type Community struct {
Name string
Description string
Popularity int
FavoriteCount int
}
The only way I can think of doing this is to create a new type containing only those fields and write a convenience method that takes a community and returns that type, but essentially creating a new object and copying those fields by value, is there a better way to do this?
I'm aware of the json:"-" syntax, but I'm not sure of how you could do this on a case by case basis as I still need to sometimes return the full object, perhaps a different type that is converted to?

Answer 1

[This](http://attilaolah.eu/2014/09/10/json-and-struct-composition-in-go/
) is a cool approach, which involves creating a sort of Masking struct.
Here's the example in the article:
type User struct {
Email string `json:"email"`
Password string `json:"password"`
// many more fields…
}
type omit *struct{}
type PublicUser struct {
*User
Password omit `json:"password,omitempty"`
}
// when you want to encode your user:
json.Marshal(PublicUser{
User: user,
})

Answer 2

I developed a library which can help you in this regard: Sheriff
You can annotate your struct fields with special tags and call Sheriff to transform the given struct into a subset of it. After that you can call json.Marshal() or whatever else you want to marshal into.
Your example would become as simple as:
type Community struct {
Name string `json:"name" groups:"trending,detail"`
Description string `json:"description" groups:"trending,detail"`
Sources []Source `json:"sources" groups:"detail"`
Popularity int `json:"popularity" groups:"trending,detail"`
FavoriteCount int `json:"favorite_count" groups:"trending,detail"`
Moderators []string `json:"moderators" groups:"detail"`
Children []Community `json:"children" groups:"detail"`
Tracks []Track `json:"tracks" groups:"detail"`
}
communities := []Community{
// communities
}
o := sheriff.Options{
Groups: []string{"trending"},
}
d, err := sheriff.Marshal(&o, communities)
if err != nil {
panic(err)
}
out, _ := json.Marshal(d)

Answer 3

Yep that is the only way as far as I know using the default marshaler. The only other option is if you create your own json.Marshaler .
type Community struct {
}
type CommunityShort Community
func (key *Community) MarshalJSON() ([]byte, os.Error) {
...
}
func (key *Community) UnmarshalJSON(data []byte) os.Error {
...
}
func (key *CommunityShort) MarshalJSON() ([]byte, os.Error) {
...
}
func (key *CommunityShort) UnmarshalJSON(data []byte) os.Error {
...
}

Answer 4

I'll present you another approach that I've developed. I think it's much more clean. The only downside is slightly complicated object initialization, but in usage it's very streamlined.
The main point is that you're not basing your JSON-view-object on the original object and then hiding elements in it, but the other way around, making it a part of the original object:
type CommunityBase struct {
Name string
Description string
}
type Community struct {
CommunityBase
FavoriteCount int
Moderators []string
}
var comm = Community{CommunityBase{"Name", "Descr"}, 20, []string{"Mod1","Mod2"}}
json.Marshal(comm)
//{"Name":"Name","Description":"Descr","FavoriteCount":20,"Moderators":["Mod1","Mod2"]}
json.Marshal(comm.CommunityBase)
//{"Name":"Name","Description":"Descr"}
And that's all if you need only one view, or if your views are gradually expanded.
But if your views can't be inherited, you'll have to resort to a kind of mixins, so you can make a combined view from them:
type ThingBaseMixin struct {
Name string
}
type ThingVisualMixin struct {
Color string
IsRound bool
}
type ThingTactileMixin struct {
IsSoft bool
}
type Thing struct {
ThingBaseMixin
ThingVisualMixin
ThingTactileMixin
Condition string
visualView *ThingVisualView
tactileView *ThingTactileView
}
type ThingVisualView struct {
*ThingBaseMixin
*ThingVisualMixin
}
type ThingTactileView struct {
*ThingBaseMixin
*ThingTactileMixin
}
func main() {
obj := Thing {
ThingBaseMixin: ThingBaseMixin{"Bouncy Ball"},
ThingVisualMixin: ThingVisualMixin{"blue", true},
ThingTactileMixin: ThingTactileMixin{false},
Condition: "Good",
}
obj.visualView = &ThingVisualView{&obj.ThingBaseMixin, &obj.ThingVisualMixin}
obj.tactileView = &ThingTactileView{&obj.ThingBaseMixin, &obj.ThingTactileMixin}
b, _ := json.Marshal(obj)
fmt.Println(string(b))
//{"Name":"Bouncy Ball","Color":"blue","IsRound":true,"IsSoft":false,"Condition":"Good"}
b, _ = json.Marshal(obj.ThingVisualMixin)
fmt.Println(string(b))
//{"Color":"blue","IsRound":true}
b, _ = json.Marshal(obj.visualView)
fmt.Println(string(b))
//{"Name":"Bouncy Ball","Color":"blue","IsRound":true}
b, _ = json.Marshal(obj.tactileView)
fmt.Println(string(b))
//{"Name":"Bouncy Ball","IsSoft":false}
}
Here I've added a view into the object, but if you like, you can create it just when calling Marshal:
json.Marshal(ThingVisualView{&obj.ThingBaseMixin, &obj.ThingVisualMixin})
Or even without a preliminary type declaration:
json.Marshal(struct{*ThingBaseMixin;*ThingVisualMixin}{&obj.ThingBaseMixin,&obj.ThingVisualMixin})

Answer 5

Not sure why this isn't the preferred method, maybe due to the age of the post, but as far as I know, this is the 'best practice' way to handle this, with 'omitempty' tags for those which don't have to exist in the JSON object.
type Community struct {
Name string `json:"name"`
Description string `json:"description"`
Sources *[]Source `json:"sources,omitempty"`
Popularity int `json:"popularity"`
FavoriteCount int `json:"favorite-count"`
Moderators *[]string `json:"moderators,omitempty"`
Children *[]Community `json:"children,omitempty"`
Tracks *[]Track `json:"tracks,omitempty"`
}