How to Flatten JSON Output from a Go struct containing sql.NullString?

Marshaling sql.NullString for Flattened Output

Problem

Given a Go struct containing a sql.NullString, marshaling the struct using encoding/json produces a nested object instead of the desired flattened value.

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

For instance, marshaling such a struct results in output like:

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

However, the desired output is flattened:

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

Solution

Customizing the MarshalJSON method for the NullString type by defining a new type allows for a more controlled output.

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"`
}

By implementing the json.Marshaler interface, the MyNullString type can control how it is represented as JSON. When it is non-null, it returns the underlying string, and for null values, it returns a literal "null" string.

The example provided in the code playground demonstrates the desired behavior:

https://play.golang.org/p/Ak_D6QgIzLb

This solution flattens the output by ignoring the Valid field and returning the string value directly.

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