Invalid filter add_class using django-widget-tweaks

Question

I am creating form view using django-widget-tweaks following this tutorial.
When I tried to implement add_class filter, I got following error.

Invalid filter: 'add_class'

Does anyone know how to solve this error?

html page

{% extends "base.html" %}
(% load widget_tweaks %}

{% block content %}
    <form method="post" enctype="multipart/form-data">
        <h4 style="margin-top: 0">Project Upload</h4>
        {% csrf_token %}
        {% for hidden in form.hidden_fields %}
            {{hidden}}
        {% endfor %}

        {% for field in form.visible_fields %}
            <div class="form-group">
                <label for="{{field.id_for_label}}">{{field.label}}</label>
                {{field|add_class:"form-control"}}
            </div>
        {% endfor %}

        <button type="submit">Upload</button>
    </form>
{% endblock %}

form.py

class DocumentForm(forms.ModelForm):

    class Meta:
        model=html
        fields=['project','version','diff','program','location']

Answer 1

You can check this-out here

{% load widget_tweaks %}

<!-- add 2 extra css classes to field element -->
{{ form.title|add_class:"css_class_1 css_class_2" }}

Answer 2

Try this :

{% render_field field|add_class:"form-group"%}