title: Django 2.0 url parameters in get_queryset
body: <p>I would like to filter subcategories based on the category id from the URL</p>
  <p>For a constant value, it works without a problem</p>
  <pre><code>return Subcategory.objects.filter(category = 1)
  </code></pre>
  <p>views.py</p>
  <pre><code>class SubcategoriesListView(ListView):
      model = Subcategory
      template_name = 'app/categories/index.html'
      def get_queryset(self):
          return Subcategory.objects.filter(category = category_id)
  </code></pre>
  <p>urls.py</p>
  <pre><code>path('categories/&lt;int:category_id&gt;/', app.views.SubcategoriesListView.as_view(), name='subcategories'),
  </code></pre>
  <p>models.py</p>
  <pre><code>class Subcategory(models.Model):
     title = models.CharField(max_length=30)
     category = models.ForeignKey(Category, on_delete=models.CASCADE)
  </code></pre>
  <p>Traceback</p>
  <pre><code>NameError at /categories/1/
  name 'category_id' is not defined
  
  views.py in get_queryset
  return Subcategory.objects.filter(category = category_id) 
  </code></pre>