Assigning null to JSON fields instead of empty strings

Question

Since empty string is the zero/default value for Go string, I decided to define all such fields as interface{} instead. for example

Answer 1

Another way actually is a workaround with using the MarhshalJSON and UnmarshalJSON interface method offered by the json lib of golang. The code is as below:

type MyType string
type MyStruct struct {
    A MyType `json:"my_type"`
}

func (c MyType) MarshalJSON() ([]byte, error) {
    var buf bytes.Buffer
    if len(string(c)) == 0 {
        buf.WriteString(`null`)
    } else {
        buf.WriteString(`"` + string(c) + `"`)   // add double quation mark as json format required
    }
    return buf.Bytes(), nil
}

func (c *MyType)UnmarshalJSON(in []byte) error {
    str := string(in)
    if str == `null` {
        *c = ""
        return nil
    }
    res := MyType(str)
    if len(res) >= 2 {
        res = res[1:len(res)-1]     // remove the wrapped qutation
    }
    *c = res
    return nil
}

then when using json.Marshal, the MyType value will be marshaled as null.

Answer 2

Can be used https://github.com/guregu/null

type student struct {
FirstName  null.String `json:"first_name"`
MiddleName null.String `json:"middle_name"`
LastName   null.String `json:"last_name"`}

Answer 3

You could use something like sql.NullString,use 'Valid' to check if it is a nil value.

NullString represents a string that may be null. NullString implements the Scanner interface so it can be used as a scan destination:

type NullString struct {
    String string
    Valid  bool // Valid is true if String is not NULL
}

var s NullString
err := db.QueryRow("SELECT name FROM foo WHERE id=?", id).Scan(&s)
...
if s.Valid {
   // use s.String
} else {
   // NULL value
}

Please DO NOT use pointers like below:

type student struct {
  FirstName  *string `json:"first_name"`
  MiddleName *string `json:"middle_name"`
  LastName   *string `json:"last_name"`
}

Because you need check nil value and dereference like below everywhere,and maybe cause unexpected crash somewhere:

if obj.FirstName != nil {
   fmt.Print("%s", *obj.FirstName)
}

I have compared two solution and choosed former method in my extensively production code,it work ok.

Answer 4

In json package documentation :

Pointer values encode as the value pointed to. A nil pointer encodes as the null JSON object.

So you can store a pointer to a string which will be encoded as a string if not nil and will be encoded as "null" if nil

type student struct {
  FirstName  *string `json:"first_name"`
  MiddleName *string `json:"middle_name"`
  LastName   *string `json:"last_name"`
}

Answer 5

For the case of a json object with null strings, its easiest to use the omitempty decorator on the field.

type student struct {
  FirstName  string `json:"first_name,omitempty"`
  MiddleName string `json:"middle_name,omitempty"`
  LastName   string `json:"last_name"`
}

With the above declaration, only if first_name was assigned will that key show up in the resultant json. last_name, on the otherhand, will always show up in the result with a value of "" if not assigned.

Now when you start including numeric fields where 0 could be a value, using omitempty doesn't do what one would expect. A 0 value always drops the field, and we need to be able to differentiate between a 0 value and an unassigned value. Here use of library such as https://github.com/guregu/null may be advisable.

More discussion here: https://www.sohamkamani.com/blog/golang/2018-07-19-golang-omitempty/