How can I work with SQL NULL values and JSON in a good way?

Go types like Int64 and String cannot store null values, so I found I could use sql.NullInt64 and sql.NullString for this.

But when I use these in a Struct, and generate JSON from the Struct with the json package, then the format is different to when I use regular Int64 and String types.

The JSON has an additional level because the sql.Null*** is also a Struct.

Is there a good workaround for this, or should I not use NULLs in my SQL database?

Types like sql.NullInt64 do not implement any special handling for JSON marshaling or unmarshaling, so the default rules apply. Since the type is a struct, it gets marshalled as an object with its fields as attributes.

One way to work around this is to create your own type that implements the json.Marshaller / json.Unmarshaler interfaces. By embedding the sql.NullInt64 type, we get the SQL methods for free. Something like this:

type JsonNullInt64 struct {
	sql.NullInt64
}

func (v JsonNullInt64) MarshalJSON() ([]byte, error) {
    if v.Valid {
		return json.Marshal(v.Int64)
	} else {
		return json.Marshal(nil)
	}
}

func (v *JsonNullInt64) UnmarshalJSON(data []byte) error {
	// Unmarshalling into a pointer will let us detect null
	var x *int64
	if err := json.Unmarshal(data, &x); err != nil {
		return err
	}
	if x != nil {
		v.Valid = true
		v.Int64 = *x
	} else {
		v.Valid = false
	}
	return nil
}

If you use this type in place of sql.NullInt64, it should be encoded as you expect.

You can test this example here: http://play.golang.org/p/zFESxLcd-c

If you use the null.v3 package, you won't need to implement any of the marshal or unmarshal methods. It's a superset of the sql.Null structs and is probably what you want.

package main

import "gopkg.in/guregu/null.v3"

type Person struct {
	Name     string      `json:"id"`
	Age      int         `json:"age"`
	NickName null.String `json:"nickname"` // Optional
}

If you'd like to see a full Golang webserver that uses sqlite, nulls, and json you can consult this gist.