Map JSON array value to struct specific variable

Question

Let's say I have this JSON structure:

{
"name":"repo",
"tags":["1.0","2.0","3.0"]
}

And I would like to map it to this Go struct:

type Repository struct {
    Name string `json:"name"`
    Tags []struct {
        Tag   string `json:"??"`
        Sha256 string
    }
}

How can I link the "tags" array JSON value to a struct field?

EDIT: The idea will be to access the tags array value like this repository.Tags[0].Tag.

but I want to map the value to a struct Tag value. So I can call it like repository.Tags[0].tag — dsncode
– dsncode, Commented Mar 15, 2018 at 7:43

Peter · Accepted Answer · 2018-03-15 07:48:22Z

6

Implement json.Unmarshaler on a Tag type:

package main

import (
    "encoding/json"
    "log"
)

type Repository struct {
    Name string
    Tags []Tag
}

type Tag struct {
    Tag    string
    Sha256 string
}

func (t *Tag) UnmarshalJSON(b []byte) error {
    var s string

    if err := json.Unmarshal(b, &s); err != nil {
            return err
    }

    t.Tag = s
    return nil
}

func main() {
    b := []byte(`{ "name":"repo", "tags":["1.0","2.0","3.0"] }`)

    var r Repository
    err := json.Unmarshal(b, &r)
    if err != nil {
            log.Fatal(err)
    }

    log.Printf("%+v\n", r)
}

Try it on the playground: https://play.golang.org/p/ExwWhis0w0V

Marshaling back to JSON is left as an exercise for the reader.

answered Mar 15, 2018 at 7:48

Peter

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5 Comments

dsncode Over a year ago

super, it works! But I was wondering if there was a "out of the box" approach. like adding json:"" or something like that.

Himanshu Over a year ago

@dsncode you are trying to unmarshal an array into struct which don't have keys. You needs to tell the json Unmarshaler how to map array values to struct

Peter Over a year ago

@dsncode, no, there is not. The stdlib can only map JSON documents to Go types one-to-one, if that makes sense. Everything else has to be done with the json.Marshaler and json.Unmarshaler interfaces.

Gujarat Santana Over a year ago

@Peter this is interesting, and I was wondering how and where this UnmarshalJSON called ?

Peter Over a year ago

@GujaratSantana, the encoding/json package calls it when unmarshaling a value of type Tag: golang.org/pkg/encoding/json/#Unmarshaler

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