How to Flatten the Output of a Marshalled sql.NullString in Go?

How to Flatten and Output Only Value from Marshalled sql.NullString in Go?

Given a Golang struct with a sql.NullString field, the default marshaling behavior results in an output where the field is nested inside a JSON object with String and Valid keys. To achieve a flattened output containing only the value of the NullString, follow these steps:

1. Define a Custom Type:

Create a custom type that embeds sql.NullString and implements the json.Marshaler interface.

type MyNullString struct {
    sql.NullString
}

2. Implement the MarshalJSON Method:

Inside the MyNullString, implement the MarshalJSON method to provide custom JSON marshalling behavior.

func (s MyNullString) MarshalJSON() ([]byte, error) {
    if s.Valid {
        return json.Marshal(s.String)
    }
    return []byte(`null`), nil
}

3. Modify the Struct:

Use the custom MyNullString type for the field in the struct that requires flattening.

type Company struct {
    ID   int          `json:"id"`              
    Abn  MyNullString `json:"abn,string"`
}

Example:

package main

import (
    "database/sql"
    "encoding/json"
    "log"
)

type MyNullString struct {
    sql.NullString
}

func (s MyNullString) MarshalJSON() ([]byte, error) {
    if s.Valid {
        return json.Marshal(s.String)
    }
    return []byte(`null`), nil
}

type Company struct {
    ID   int          `json:"id"`              
    Abn  MyNullString `json:"abn,string"`
}

func main() {
    company := Company{
        ID:   68,
        Abn:  MyNullString{String: "SomeABN", Valid: true},
    }

    result, err := json.Marshal(company)
    if err != nil {
        log.Fatal(err)
    }

    // Output: {"id":68,"abn":"SomeABN"}
    log.Println(string(result))
}

By following these steps, you can effectively flatten the output of sql.NullString fields and obtain only the value in your marshalled JSON.

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