Unmarshal an array nested inside an JSON object - go

I am having some problems unmarshalling this JSON data to a Go slice of items that holds item structs:
response := {
"data": [
{
"name": "a",
"products": [
{
"name": "c"
}
]
},
{
"name": "b",
"products": [
{
"name": "d"
}
]
},
{
"name": "c",
"products": [
{
"name": "e"
}
]
}
]
}
These are my structs:
type Item struct {
Name string
Products
}
type Products struct {
Name string
}
The slice should basically be the value the "data" attribute (which is an array) transformed to a Go Items slice. I tried the following, but I kept getting an empty array
var items []Item{}
json.Unmarshal(response, &items)
fmt.Println(items)

You're trying to unmarshal that JSON into a slice, but the root of that JSON is an object (you can tell because the JSON begins with a {, indicating the start of an object). You need to account for that outer layer:
type Payload struct {
Data []Item
}
var payload Payload
json.Unmarshal(response, &payload)
fmt.Println(payload.Data)

I would recommend using this cool tool to help you design your struct
https://mholt.github.io/json-to-go/
Also as Adrian said you're trying to unmarshaling json with an array to an object.
type Product struct {
Name string `json:"name"`
}
type Response struct {
Data []struct {
Name string `json:"name"`
Products []Product `json:"products"`
} `json:"data"`
}

Related

How to get the number of items from a structure sub field using reflect in go

I have some issues when getting the number of items from a sub field in a slice struct through reflect package.
This is how I'm trying to get the number of items from Items
func main() {
type Items struct {
Name string `json:"name"`
Present bool `json:"present"`
}
type someStuff struct {
Fields string `json:"fields"`
Items []Items `json:"items"`
}
type Stuff struct {
Stuff []someStuff `json:"stuff"`
}
some_stuff := `{
"stuff": [
{
"fields": "example",
"items": [
{ "name": "book01", "present": true },
{ "name": "book02", "present": true },
{ "name": "book03", "present": true }
]
}
]
}`
var variable Stuff
err := json.Unmarshal([]byte(some_stuff), &variable)
if err != nil {
panic(err)
}
//I want to get the number of items in my case 3
NumItems := reflect.ValueOf(variable.Stuff.Items)
}
This is the error:
variable.Items undefined (type []Stuff has no field or method Items)
I'm unsure if I can retrieve the number of items like that.
I have already fixed the issue.
In order to get the number of sub fields we can make use of Len() from reflect.ValueOf.
The code now is getting the number of Items:
package main
import (
"encoding/json"
"fmt"
"reflect"
)
func main() {
type Items struct {
Name string `json:"name"`
Present bool `json:"present"`
}
type someStuff struct {
Fields string `json:"fields"`
Items []Items `json:"items"`
}
type Stuff struct {
Stuff []someStuff `json:"stuff"`
}
some_stuff := `{
"stuff": [
{
"fields": "example",
"items": [
{ "name": "book01", "present": true },
{ "name": "book02", "present": true },
{ "name": "book03", "present": true }
]
}
]
}`
var variable Stuff
err := json.Unmarshal([]byte(some_stuff), &variable)
if err != nil {
panic(err)
}
//I want to get the number of items in my case 3
t := reflect.ValueOf(variable.Stuff[0].Items)
fmt.Println(t.Len())
}
Output: 3

How to get/extract values of a JSON

I'm trying to get values/index/key of a JSON but no success. I found some answers and tutorials but I couldn't get them working for me.
I have the following JSON:
{
"id": "3479",
"product": "Camera",
"price": "",
"creation": 04032020,
"products": [
{
"camera": "Nikon",
"available": true,
"freeshipping": false,
"price": "1,813",
"color": "black"
},
{
"camera": "Sony",
"available": true,
"freeshipping": true,
"price": "931",
"color": "black"
}
],
"category": "eletronics",
"type": "camera"
}
I have tried several examples but none worked for this type of Json.
The error I'm getting:
panic: interface conversion: interface {} is nil, not
map[string]interface {}
I believe it's because of "products[]" I tried map[string]interface{} and []interface{} it compiles but gives me that error afterwards.
Could you give me an example on how I can extract these values?
The code I'm using:
//gets the json
product,_:=conn.GetProductData(shop.Info.Id)
// assing a variable
productInformation:=<-product
//prints the json
fmt.Printf(productInformation)
//try to get json values
type Product struct {
Id string
Product string
}
var product Product
json.Unmarshal([]byte(productInformation), &product)
fmt.Printf("Id: %s, Product: %s", product.Id, product.Product)
This code does not panic but it doesn't print all results either so I tried
this one below (which was supposed to give me all results) but it panics
var result map[string]interface{}
json.Unmarshal([]byte(productInformation), &result)
// The object stored in the "birds" key is also stored as
// a map[string]interface{} type, and its type is asserted from
// the interface{} type
products := result["products"].(map[string]interface{})
for key, value := range products {
// Each value is an interface{} type, that is type asserted as a string
fmt.Println(key, value.(string))
}
You need to add json tag to specify the field name in json as it is in lowercase
type Product struct {
Id string `json:"id"`
Product string `json:"product"`
}
And in the second case according to json it is a slice not a map so you need to cast it to []interface{}
var result map[string]interface{}
json.Unmarshal([]byte(productInformation), &result)
// The object stored in the "birds" key is also stored as
// a map[string]interface{} type, and its type is asserted from
// the interface{} type
products := result["products"].([]interface{})
for key, value := range products {
// Each value is an interface{} type, that is type asserted as a string
fmt.Println(key, value)
}

How do you convert specific array values in a JSON response to be just the first value in the array?

(Note, Dgraph isn't required for this problem; it's just the tool I am using where I am encountering the problem)
Let's say you have some structs
type AccountSettings struct {
Uid string `json:"uid"`
DType string `json:"dgraph.type"`
Name string `json:"name"`
Username string `json:"username"`
}
type Settings struct {
Uid string `json:"uid"`
DType string `json:"dgraph.type"`
Account AccountSettings `json:"account"`
}
type Tag struct {
Uid string `json:"uid"`
DType string `json:"dgraph.type"`
Label Label `json:"label"`
}
type User struct {
Uid string `json:"uid"`
DType string `json:"dgraph.type"`
Settings Settings `json:"settings"`
Tags []Tag `json:"tags"`
}
and you create a user and add it to your Dgraph database
user = User{
DType: "User",
Settings: Settings{
DType: "Settings",
Account: SettingsAccount{
DType: "SettingsAccount",
Name: "Billy Bob"
Username: "billyboy123"
},
Tags: []Tag{{
DType: "Tag",
Label: "admin"
}, {
DType: "Tag",
Label: "user"
}}
},
}
And then you query your Dgraph database like (assuming you have the uid of the user node)
{
user(func: uid("0x1234")) {
uid
settings {
account {
name
username
}
}
tags {
label
}
}
}
the response will come back as
{
"data": {
"user": [
{
"uid": "0x1234"
"settings": [
{
"account": [
{
"name": "Billy Bob",
"username": "billyboy123"
}
]
}
]
"tags": [
{
label: "admin"
},
{
label: "user"
},
]
}
]
}
}
So the problem is that the JSON response of Dgraph will return fields of struct types as arrays (presumably because there could potentially be multiple nodes pointing to a single other node), but since it's an array, you can't immediately marshal it with the User struct (because User.Settings is of type Settings, not []Settings).
What would you do in order to marshal the Dgraph JSON response with the User struct (or any other kind of struct)?
It's important to note that the settings array in the JSON response should just be the first element in that array, but the tags array should still remain an array because that's what's specified in the User struct.
You can do this with a custom Unmarshal as described here:
http://choly.ca/post/go-json-marshalling/
You use an alias by embedding the original User in an anonymous type that has a slice of Settings inside it that you Umarshal into like so:
func (u *User) UnmarshalJSON(data []byte) error {
type Alias User
aux := &struct {
Settings []*Settings `json:"settings"`
*Alias
}{
Alias: (*Alias)(u),
}
if err := json.Unmarshal(data, &aux); err != nil {
return err
}
u.Settings = aux.Settings[0]
return nil
}
A full working example is here (you need the same trick for the AccountSettings):
https://play.golang.org/p/FQWakX8B7OF

How to handle nil struct variable in struct type

I need to marshal/unmarshal json to struct in golang. Assume the struct is
type A struct {
Id string `json:"id"`
Version string `json:"version"`
Actors []actor `json:"actors`
Payload struct {
Name string `json:"name"`
Number string `json:"number"`
}
}
type payload struct {
Name string `json:"name"`
Number string `json:"number"`
}
type actor struct {
Id string `json:"id"`
Type string `json:"type"`
Role string `json:"role"`
}
The actors or payload maybe empty. The json maybe
{
"id": "78a07cea-be2b-499c-b82b-e4f510260484",
"version": "1.0.0",
"actors": [
{
"id": "1234567",
"type": "XXX",
"role": "111"
},
{
"id": "7654321",
"type": "YYY",
"role": "222"
}
],
"payload": ""
}
or
{
"id": "78a07cea-be2b-499c-b82b-e4f510260484",
"version": "1.0.0",
"actors": [],
"payload": {
"name": "XXXX",
"number": "1234567"
}
}
If i follow the struct A design and try to marshal json with payload empty, i have to init as below
a := A{
Id: "78a07cea-be2b-499c-b82b-e4f510260484",
Version: "1.0.0",
Actors: []actor{
actor{
Id: "1234567",
Type: "XXX",
Role: "111",
},
actor{
Id: "7654321",
Type: "YYY",
Role: "222",
},
},
Payload: payload{},
}
Which will result in below json with one empty payload struct
{
"id": "78a07cea-be2b-499c-b82b-e4f510260484",
"version": "1.0.0",
"actors": [
{
"id": "1234567",
"type": "XXX",
"role": "111"
},
{
"id": "7654321",
"type": "YYY",
"role": "222"
}
],
"payload": {
"name":"",
"number":""
}
}
Is there any way i can generate
"payload": ""
instead of blank payload struct? Or is there any other struct design for this kind of json format? BTW i cannot pass nil to Payload struct.
The json.Marshaler interface can be implemented to customize JSON encoding, and the json.Unmarshaler interface for decoding (left as an exercise for the reader):
package main
import (
"encoding/json"
"fmt"
)
type A struct {
Payload payload
}
type payload struct {
Name string `json:"name"`
Number string `json:"number"`
}
func (p payload) MarshalJSON() ([]byte, error) {
if p.Name == "" && p.Number == "" {
return []byte(`""`), nil
}
type _payload payload // prevent recursion
return json.Marshal(_payload(p))
}
func main() {
var a A
b, _ := json.MarshalIndent(a, "", " ")
fmt.Println(string(b))
a.Payload.Name = "foo"
b, _ = json.MarshalIndent(a, "", " ")
fmt.Println(string(b))
}
// Output:
// {
// "Payload": ""
// }
// {
// "Payload": {
// "name": "foo",
// "number": ""
// }
// }
Try it on the playground: https://play.golang.org/p/9jhSWnKTnTf
The ad-hoc _payload type is required to prevent recursion. If one would write return json.Marshal(p), the json package would call MarshalJSON again, because p is of type payload, and payload implements json.Marshaler.
The _payload type has the same underlying type as payload but does not implement json.Marshaler (see Type definitions for details), so it is encoded using the standard rules of the json package; it produces exactly the same output that encoding a value of type payload would produce if payload didn't implement json.Marshaler.
Check if using omitempty with json struct tag helps. I think it will result in "payload": {} instead of "payload": ""

Golang Facebook response to struct

Hi there I'm new in GO and I'm trying to convert a json from the facebook api to struct.
The problem is that the keys of the object are dinamic:
{
"100555213756790": {
"id": "100555213756790",
"about": "Noodle Bar & Restaurant",
"metadata": {
"fields": [
{
"name": "id",
"description": "asdasdasdasd",
"type": "numeric string"
},
//...
,
"101285033290986": {
"id": "101285033290986",
"about": "Smart City Expo World Congress",
"metadata": {
"fields": [
{
"name": "id",
"description": "fgddgdfgdg",
"type": "numeric string"
},
what I have achieved so far is extract the objects by id and turn them into a map:
for _, id := range ids {
fbPages, ok := results[string(id)].(map[string]interface{})
if ok {
for k, v := range fbPages {
fmt.Println(k)
fmt.Println(v)
}
}
}
//json to Page struct?
type Page struct {
ID string `json:"id"`
About string `json:"about"`
}
type Metadata struct {
Fields []Field `json:"fields"`
Type string `json:"type"`
Connections map[string]string `json:"connections"`
}
type Field struct {
Name string `json:"name"`
Description string `json:"description"`
Type *string `json:"type,omitempty"`
}
My question is:
how can I convert that map to struct? or is there any easy way to do what I'm trying to do?
Thank you
Converting map to struct:
import "github.com/mitchellh/mapstructure"
mapstructure.Decode(myMap, &myStruct)
example
But I would do this:
type Page struct {
ID string `json:"id"`
About string `json:"about"`
//other fields and nested structs like your metadata struct
}
type fbPages map[string]Page

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