title: Django-Filter: Creating checkboxes for Boolean Field
body: <p><strong>Models.py</strong></p>
  
  <pre><code>class Task(models.Model):
      online = models.BooleanField(blank=False)
  </code></pre>
  
  <p>I would like to use django-filter to create the following checkboxes in the form:</p>
  
  <p>[] Online
  [] Physical</p>
  
  <p>If form is empty or both are ticked, get <code>Task.objects.all()</code>. If only one is ticked, then do <code>Task.objects.filter('Online'=True/False)</code>.</p>
  
  <p>I tried to add the following:</p>
  
  <pre><code>import django_filters
  from app.models import Task
  from django.db import models
  
  class TaskFilter(django_filters.FilterSet):
      online = django_filters.BooleanFilter(name='online', lookup_expr='isnull')
  
      class Meta:
          model = Task
          fields = ['online']
          filter_overrides = {
              models.BooleanField: { 
                  'filter_class': django_filters.BooleanFilter,
                  'extra': lambda f: {
                      'widget': forms.CheckboxInput,
                  },
              },
          }
  </code></pre>
  
  <p>I tried the following widgets: <code>Select, RadioSelect, CheckboxInput</code>, but it seems I get the same output for all of them. I get a dropdown list e.g. </p>
  
  <p>Online: [ Dropdown to select a value from Yes, No, Unknown ]</p>