How to Flatten sql.NullString Output When Marshaling JSON in Go?

Flattening sql.NullString Output During JSON Marshaling

When marshaling a Go struct containing an sql.NullString field, the default output includes the field's Valid and String properties. To achieve a flattened output that only includes the value, you must take additional steps.

Consider the following struct:

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

Marshalling this struct with json.Marshal produces an output that looks like:

{
    "id": "68",
    "abn": {
        "String": "SomeABN",
        "Valid": true
    }
}

Extend sql.NullString for Custom JSON Marshaling

To flatten this output, you must extend sql.NullString and implement json.Marshaler.

type NullString sql.NullString

func (x *NullString) MarshalJSON() ([]byte, error) {
    if !x.Valid {
        x.Valid = true
        x.String = ""
        //return []byte("null"), nil
    }
    return json.Marshal(x.String)
}

Define a Custom Type for Flattened Output

Next, define a custom type that embeds the extended NullString and implements json.Marshaler.

type MyNullString struct {
    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"`
}

By using this custom type, the marshaled output becomes:

{
    "id": "68",
    "abn": "SomeABN"
}

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