Unmarshalling sql.NullTime structures in Go

php Editor Xiaoxin In the Go language, we often use the sql.NullTime structure to process the time field in the database. The NullTime structure can represent a nullable time value and is very suitable for handling null values ​​in the database. In this article, we will explain how to unmarshal a sql.NullTime structure and how to correctly handle null situations that may exist within it. Whether you are a beginner or an experienced developer, this article will provide you with clear guidance to help you better understand and use the NullTime structure.

Question content

Given

type NullTime struct {
    Time  time.Time
    Valid bool // Valid is true if Time is not NULL
}

and

type PayinCount struct {
    DateShiftStart sql.NullTime    `json:"dateShiftStart"`
    DateShiftEnd   sql.NullTime    `json:"dateShiftend"`
}

When I process the following JSON

{
    "dateShiftStart":"2023-10-16",
    "dateShiftEnd":"2023-10-23"
}

and

var payinsCount PayinsCount
err = json.Unmarshal(body, &payinsCount)
if err != nil {
    sendErrorResponse(w, err.Error(), http.StatusBadRequest)
    return
}

Where sendErrorResponse is the following auxiliary process

func sendErrorResponse(w http.ResponseWriter, err string, statusCode int) {
    messageStatusCode := MessageStatusCode{
        Message:    err,
        StatusCode: statusCode}
    w.WriteHeader(statusCode)
    json.NewEncoder(w).Encode(messageStatusCode)
}

I received the following message

{
    "message": "json: cannot unmarshal string into Go struct field PayinsCount.dateShiftStart of type sql.NullTime",
    "statusCode": 400
}

how to solve this problem?

Workaround

I ended up using the following. I added the following types.

type NullDate sql.NullTime

Then I changed the PayinsCount to use NullDate

type PayinsCount struct {
    DateShiftStart NullDate      `json:"dateShiftStart,omitempty"`
    DateShiftEnd   NullDate      `json:"dateShiftend,omitempty"`
}

Then I created

// UnmarshalJSON for NullDate
func (nd *NullDate) UnmarshalJSON(b []byte) error {
    s := string(b)
    s = strings.ReplaceAll(s, "\"", "")

    x, err := time.Parse(time.DateOnly, s)
    if err != nil {
        nd.Valid = false
        return err
    }

    nd.Time = x
    nd.Valid = true
    return nil
}

Now when I process the following JSON

{
    "dateShiftStart":"2023-10-16",
    "dateShiftEnd":"2023-10-23"
}

and

var payinsCount PayinsCount
err = json.Unmarshal(body, &payinsCount)
if err != nil {
    sendErrorResponse(w, err.Error(), http.StatusBadRequest)
    return
}

It works. I ended up with a valid PayinsCount instance.

For completeness, here is the MarshalJSON function for NullDate

// MarshalJSON for NullDate
func (nd NullDate) MarshalJSON() ([]byte, error) {
    if !nd.Valid {
        return []byte("null"), nil
    }
    val := fmt.Sprintf("\"%s\"", nd.Time.Format(time.DateOnly))
    return []byte(val), nil
}

Note the escaped double quotes - without them the encoding/json marshaling code processes the date string in 3 chunks and I get the following error

error(*encoding/json.SyntaxError) *{msg: "invalid character '-' after top-level value", Offset: 0}

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