I don't know if ActiveRecord is capable of doing this, but here is how to do it with Django ORM:

in models.py:

    class Node(models.Model):
        parent = models.ForeignKey('self', related_name='children')

in the view:

    nodes = Node.objects.prefetch_related('children')

in template:

    {% for node in nodes %}
        {% for subnode in node.children.all %}
        ...
        {% endfor %}
    {% endfor %}

This will make exactly two queries; one for all the parent nodes, and one for the children nodes. Django will make the pairing automagically, so you don't have to do it in view code.