class Post(models.Model):
...

def list_by_category(self):
    return Post.objects.filter(category=self.category)

-----
template:
-----
<h2>Related Posts</h2>
{% if list_by_category %}
  {% for post in list_by_category %}
     <h3><a href="{{ post.get_absolute_url }}"> etc...

...end blocks, etc.

Does it need to be post.list_by_category?