How to implement the preview feature like in Toaster?

Question

Django

Hello, please help me understand. The Toaster has a preview of the question being created. I'm trying to create something similar in my Django project.
There is an article editing form with two buttons and two fields. The first button is submit, the second button is preview. One of the fields is a CharField and it stores the html markup. A wysiwyg editor has thrown on this field. The form itself is in a modal window.
There is a following task. When you click on the second button (preview), I want to send a post request via ajax to send data from the fields to Redis. Next, change the content of the form to this temporary data in order to get some kind of preview.
I'm trying to link my Django project to Redis. I use this app for this: django-redis .
settings.py:

CACHES = {
    "default": {
        "BACKEND": "django_redis.cache.RedisCache",
        "LOCATION": "redis://127.0.0.1:6379/1",
        "OPTIONS": {
            "CLIENT_CLASS": "django_redis.client.DefaultClient",
        }
    }
}

There are two questions.
1. How to write data from fields in Redis in Сlass Based View? So far, I have attached the cache_page decorator to the url. Not sure if this is best practice.
2. The second question is about ajax. I can not figure out how it should generally look in my particular case.
What I have tried so far:
template:

<form method="post" action="{% url 'article:article_edit' article.id %}" class="article-edit-form" enctype="multipart/form-data">
  {% csrf_token %}
  <a id="article-preview-btn" class="btn btn-info" data-url="{% url 'article:article_preview' article.id %}">
    <span>{% trans "Превью" %}</span>
  </a>
  <button type="submit" class="btn btn-success">{% trans 'Обновить' %}</button>
</form>

URLs.py:

from django.views.decorators.cache import cache_page

url(r'^(?P<pk>\d+)/preview/$',
     cache_page(15 * 60)(ArticlePreview.as_view()),
     name='article_preview'),

views.py:

class ArticlePreview(FormView):
    form_class = ArticleForm

    def form_valid(self, form):
        data = dict()
        data['session'] = self.request.session
        data['title'] = form.cleaned_data['title']
        data['body'] = form.cleaned_data['body']
        return JsonResponse(data)

JS:

$("#article-modal").on("click", "#article-preview-btn", function () {
    var btn = $(this);
    $.ajax({
      url: btn.attr("data-url"),
      type: 'post',
      dataType: 'json',
      success: function (data) {
        ???
      }
    });
});

ERROR:

Internal Server Error: /article/1/preview/
Traceback (most recent call last):
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35\lib\site-packages\django\core\handlers\exception.py", line 41, in inner
    response = get_response(request)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35\lib\site-packages\django\core\handlers\base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35\lib\site-packages\django\core\handlers\base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35\lib\site-packages\django\utils\decorators.py", line 149, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35\lib\site-packages\django\views\generic\base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35\lib\site-packages\django\views\generic\base.py", line 88, in dispatch
    return handler(request, *args, **kwargs)
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35\lib\site-packages\django\views\generic\edit.py", line 181, in post
    form = self.get_form()
  File "C:\Users\Nurzhan\AppData\Local\Programs\Python\Python35\lib\site-packages\django\views\generic\edit.py", line 45, in get_form
    return form_class(**self.get_form_kwargs())
TypeError: __init__() missing 1 required positional argument: 'user'
[18/Dec/2017 23:34:23] "POST /article/1/preview/ HTTP/1.1" 500 16682