페이지의 삭제 버튼을 눌렀을 때 'undefined' is not a valid ObjectId, it must be a 12-byte input or a 24-character hex string 이런 오류가 발생합니다.. 물론 웹종 관련 내용은 아니라 올리면 안되지만 몇십분째 구글링을 해도 모르겠어서 올립니다 ㅠㅠ

function listing() {
    $.ajax({
        type: "GET",
        url: "diary",
        data: {},
        success: function (response) {
            let diaries = response['all_diary']
            for (let i = 0; i < diaries.length; i++) {
                let title = diaries[i]['title']
                let content = diaries[i]['content']
                let file = diaries[i]['file']

                let temp_html = `<div class="card">
                                        <img class="card-img-top" src="../static/${file}" alt="Card image cap">
                                        <div class="card-body">
                                            <h5 class="card-title">${title}</h5>
                                            <p class="card-text">${content}</p>
                                            <p class="card-text"><small class="text-muted"></small></p>
                                        </div>
                                        <button type="button" class="btn btn-secondary" onclick="delete_diary('${diaries["_id"]}')"> 삭제 </button>
                                    </div>`
                $('#cards-box').append(temp_html)
            }
        }
    })
}

function posting() {
    let title = $('#title').val()
    let content = $("#content").val()

    let file = $('#file')[0].files[0]
    let form_data = new FormData()

    form_data.append("file_give", file)
    form_data.append("title_give", title)
    form_data.append("content_give", content)

    $.ajax({
        type: "POST",
        url: "/api/save_diary",
        data: form_data,
        cache: false,
        contentType: false,
        processData: false,
        success: function (response) {
            alert(response["msg"])
            window.location.reload()
        }
    });
}

function delete_diary(_id) {
    console.log(_id)

    let title = $('#title').val()
    let content = $("#content").val()
    let file = $('#file')[0].files[0]

    $.ajax({
        type: "POST",
        url: `/api/delete_diary`,
        data: {
            _id: _id
        },
        success: function (response) {
            alert(response["msg"])
        }
    });

}

@app.route('/api/save_diary', methods=['POST'])
def save_diary():
    title_receive = request.form['title_give']
    content_receive = request.form['content_give']

    file = request.files["file_give"]

    extension = file.filename.split('.')[-1]

    today = datetime.now()
    mytime = today.strftime('%Y-%m-%d-%H-%M-%S')

    filename = f'file-{mytime}'

    save_to = f'static/{filename}.{extension}'
    file.save(save_to)

    doc = {
        'title':title_receive,
        'content':content_receive,
        'file': f'{filename}.{extension}',
        'time': today.strftime('%Y.%m.%d')
    }

    db.diary.insert_one(doc)

    return jsonify({'msg': '저장 완료!'})

@app.route('/api/delete_diary', methods=['post'])
def delete_diary():
    _id = request.form['_id']
    print(_id)
    db.diary.delete_one({'_id': ObjectId(_id)})

    return jsonify({'result': 'success', 'msg' : '글 삭제'})