Inconsistent json unmarshal

leofigy — Tue, 24 Sep 2019 03:23:14 +0000

Common daily task

Go provides has a json package in the standard library. However a common case we face everyday at work is that our input is not consistent, that means a json entry could have different data types.

Consider the following input jsons

# good input
{
    "name": "Sam smith",
    "age" : 34
}
# lazy input 
{
    "name": "Sam smith stringer",
    "age" : "41"
}

as one can see the "age" field has two different types string and number.
Go provides a trick to handle this case; So the recipe is

Write your go struct with the right types
Create an Alias for your struct to avoid recursively unmarshal
Unmarshal to your intermediate struct with your embbeded alias
Select the attributes of your struct and parse them to the intended type

Full code snippet

package main

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

type InputJSON struct {
    Name string `json:"name,omitempty"`
    Age  int    `json:"age,omitempty"`
}

/* Alias to avoid calling implicit UnmarshalJSON of your original 
   struct 
*/
type Alias InputJSON

/* write your Unmarshal function using an intermediate struct to use interface to handle any type */
func (i *InputJSON) UnmarshalJSON(input []byte) error {
    // intermediate step
    middle := struct {
        Alias 
        Age interface{} `json:"age,omitempty"`
    }{}

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

    *i = InputJSON(middle.Alias)

    if middle.Age != nil {
        switch realValue := (middle.Age).(type) {
        case string:
            intValue, err := strconv.Atoi(realValue)
            if err != nil {
                return err
            }
            i.Age = intValue
        case float64:
            // by default uses a float64 for number representation
            i.Age = int(realValue)
        }
    }
    return nil
}

func main() {
    // use cases 
    expectedCase := []byte(`
        {
            "name": "Sam smith",
            "age" : 34
        }
    `)

    awfulCase := []byte(`
        {
            "name": "Sam smith stringer",
            "age" : "41"
        }
    `)

    goodInput := InputJSON{}

    if err := json.Unmarshal(expectedCase, &goodInput); err != nil {
        fmt.Println(err)
    }
    fmt.Printf("%+v\n", goodInput)

    badInput := InputJSON{}

    if err := json.Unmarshal(awfulCase, &badInput); err != nil {
        fmt.Println(err)
    }
    fmt.Printf("%+v\n", badInput)

}

Not the most elegant way to do it, but it works :)

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

DEV Community: leofigy

Inconsistent json unmarshal

Common daily task

Full code snippet