How do I get entries of a child category, related to a parent category, defined in a URL segment? My entry url structure looks like this:

http://site.com/section/category/subcategory/entry_slug


I'm trying to drill down to specific entries by using the url structure and I'm currently having trouble getting the following to work:

{% set secondSeg = craft.request.getSegment(2) %}
{% set subCategories = craft.categories.group('section').level(2) %}

{% for cat in subCategories.relatedTo(secondSeg).slug(lastSeg) %}
    {{category.title}}
{% endfor %}


Currently nothing is returned. If I remove the .relatedTo(secondSeg) parameter in the cat loop, I'm properly getting all of the entries in child category that match the slug. But, the problem is that there is a possibility of certain entries having the same slug under different child categories. I'm not sure if I'm using the relatedTo parameter properly or not.
I don't think that there's a "relation" between categories and their sub-categories.
But that's what you currently have as a parameter on craft.categories.
Use the ancestorOf() parameter instead to filter your subcategories:

{% set mainCategory = craft.categories.group('section').level(1).slug(secondSeg).first() %}
{% set secondLevelCategories = craft.categories.ancestorOf(mainCategory).slug(lastSeg) %}