First of all you should rename your Comments
model to Comment
, as it represent a single comment. Then you would do this:
post = Post.objects.get(pk=1)
comments = post.comment_set.all()
You can read more about reverse relationships in the documentation.
Edit: Yes, you can also do this in a template. Provided you get a post object in a variable named post
you could do something similar to this (obviously you would need to add some HTML):
{% for comment in post.comment_set.all %}
{{ comment.author }}: {{ comment.content }}
{% endfor %}