CODEWITHSUNDEEP
jsongo
user2671513
user2671513

Reputation:

Unmarshal nested JSON structure

http://play.golang.org/p/f6ilWnWTjm

I am trying to decode the following string but only getting null values.

How do I decode nested JSON structure in Go?

I want to convert the following to map data structure.

package main

import (
  "encoding/json"
  "fmt"
)

func main() {
  jStr := `
{
    "AAA": {
        "assdfdff": ["asdf"],
        "fdsfa": ["1231", "123"]
    }
}
`
  type Container struct {
    Key string `json:"AAA"`
  }
  var cont Container

  json.Unmarshal([]byte(jStr), &cont)
  fmt.Println(cont)
}

Upvotes: 12

Views: 22730

Answers (2)

Simon Fox
Simon Fox

Reputation: 6425

Use nested structs in Go to match the nested structure in JSON.

Here's one example of how to handle your example JSON:

package main

import (
    "encoding/json"
    "fmt"
    "log"
)

func main() {
    jStr := `
    {
        "AAA": {
            "assdfdff": ["asdf"],
            "fdsfa": ["1231", "123"]
        }
    }
    `

    type Inner struct {
        Key2 []string `json:"assdfdff"`
        Key3 []string `json:"fdsfa"`
    }
    type Container struct {
        Key Inner `json:"AAA"`
    }
    var cont Container
    if err := json.Unmarshal([]byte(jStr), &cont); err != nil {
        log.Fatal(err)
    }
    fmt.Printf("%+v\n", cont)
}

playground link

You can also use an anonymous type for the inner struct:

type Container struct {
    Key struct {
        Key2 []string `json:"assdfdff"`
        Key3 []string `json:"fdsfa"`
    }  `json:"AAA"`
}

playground link

or both the outer and inner structs:

var cont struct {
    Key struct {
        Key2 []string `json:"assdfdff"`
        Key3 []string `json:"fdsfa"`
    } `json:"AAA"`
}

playground link

If you don't know the field names in the inner structure, then use a map:

type Container struct {
    Key map[string][]string `json:"AAA"`
}

http://play.golang.org/p/gwugHlCPLK

There are more options. Hopefully this gets you on the right track.

Upvotes: 24

ANisus
ANisus

Reputation: 77955

First of all: Don't ignore errors returned by a function or method unless you have a very good reason to do so.

If you make the following change to the code:

err := json.Unmarshal([]byte(jStr), &cont)
if err != nil {
    fmt.Println(err)
}

You will see the error message telling you why the value is empty:

json: cannot unmarshal object into Go value of type string

Simply put: Key cannot be of type string, so you have to use a different type. You have several option on how to decode it depending on the characteristics of the Key value:

  • If the structure is static, use another struct to match this structure.
  • If the structure is dynamic, use interface{} (or map[string]interface{} if it is always of type JSON object)
  • If the value is not to be accessed, only to be used in later JSON encoding, or if the decoding is to be delayed, use json.RawMessage

Upvotes: 7

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